Issue #2135 - Implement sequential focus navigation for shadow DOM

This covers the following:
Bug 1413834	Implement sequential focus navigation regarding shadow DOM
> Bug 1430020	Let sequential focus navigation in shadow DOM enter iframes
> Bug 1430701	Handle focus navigation on frameless shadow hosts in light DOM
> Bug 1430692	Handle focus navigation on NAC in shadow DOM
> Bug 1453693	Ensure sequential focus navigation works in Shadow DOM and add some tests
> Bug 1466581	Handle sequential focus also in nested shadow DOM
> Bug 1481079	Shadow DOM hosts should be focusable
> Bug 1507101	Use StyleChildrenIterator instead of custom frame tree walking code to handle NAC inside shadow dom
> Bug 1512043	Ensure traverse all nodes owned by the top level shadow host
> Bug 1512457	Fix various cases that focus navigation doesn't work well with frameless shadow host
> Bug 1513141	Really minor nsFocusManager cleanup
> Bug 1519090	Keyboard focus is trapped inside <slot>
> Bug 1528034	Make IsHostOrSlot null-safe
> Bug 1544826	Wrong focus navigation behavior when the root element is a shadow root
>> Bug 1500273	Ensure backward focus navigation works in Shadow DOM

dom/base/nsFocusManager.h

@@ -384,6 +384,91 @@ protected:
                                        int32_t aType, bool aNoParentTraversal,
                                        nsIContent** aNextContent);
 
+  /**
+   * Returns scope owner of aContent.
+   * A scope owner is either a document root, shadow host, or slot.
+   */
+  nsIContent* FindOwner(nsIContent* aContent);
+
+  /**
+   * Retrieve the next tabbable element in scope owned by aOwner, using
+   * focusability and tabindex to determine the tab order.
+   *
+   * aOwner is the owner of scope to search in.
+   *
+   * aStartContent is the starting point for this call of this method.
+   *
+   * aOriginalStartContent is the initial starting point for sequential
+   * navigation.
+   *
+   * aForward should be true for forward navigation or false for backward
+   * navigation.
+   *
+   * aCurrentTabIndex is the current tabindex.
+   *
+   * aIgnoreTabIndex to ignore the current tabindex and find the element
+   * irrespective or the tab index.
+   *
+   * aForDocumentNavigation informs whether we're navigating only through
+   * documents.
+   *
+   * aSkipOwner to skip owner while searching. The flag is set when caller is
+   * |GetNextTabbableContent| in order to let caller handle owner.
+   *
+   * NOTE:
+   *   Consider the method searches downwards in flattened subtree
+   *   rooted at aOwner.
+   */
+  nsIContent* GetNextTabbableContentInScope(nsIContent* aOwner,
+                                            nsIContent* aStartContent,
+                                            nsIContent* aOriginalStartContent,
+                                            bool aForward,
+                                            int32_t aCurrentTabIndex,
+                                            bool aIgnoreTabIndex,
+                                            bool aForDocumentNavigation,
+                                            bool aSkipOwner);
+
+  /**
+   * Retrieve the next tabbable element in scope including aStartContent
+   * and the scope's ancestor scopes, using focusability and tabindex to
+   * determine the tab order.
+   *
+   * aStartOwner is the scope owner of the aStartContent.
+   *
+   * aStartContent an in/out paremeter. It as input is the starting point
+   * for this call of this method; as output it is the shadow host in
+   * light DOM if the next tabbable element is not found in shadow DOM,
+   * in order to continue searching in light DOM.
+   *
+   * aOriginalStartContent is the initial starting point for sequential
+   * navigation.
+   *
+   * aForward should be true for forward navigation or false for backward
+   * navigation.
+   *
+   * aCurrentTabIndex returns tab index of shadow host in light DOM if the
+   * next tabbable element is not found in shadow DOM, in order to continue
+   * searching in light DOM.
+   *
+   * aIgnoreTabIndex to ignore the current tabindex and find the element
+   * irrespective or the tab index.
+   *
+   * aForDocumentNavigation informs whether we're navigating only through
+   * documents.
+   *
+   * NOTE:
+   *   Consider the method searches upwards in all shadow host- or slot-rooted
+   *   flattened subtrees that contains aStartContent as non-root, except
+   *   the flattened subtree rooted at shadow host in light DOM.
+   */
+  nsIContent* GetNextTabbableContentInAncestorScopes(nsIContent* aStartOwner,
+                                                     nsIContent** aStartContent,
+                                                     nsIContent* aOriginalStartContent,
+                                                     bool aForward,
+                                                     int32_t* aCurrentTabIndex,
+                                                     bool aIgnoreTabIndex,
+                                                     bool aForDocumentNavigation);
+
   /**
    * Retrieve the next tabbable element within a document, using focusability
    * and tabindex to determine the tab order. The element is returned in
@@ -503,6 +588,16 @@ private:
 
   void SetFocusedWindowInternal(nsPIDOMWindowOuter* aWindow);
 
+  bool TryDocumentNavigation(nsIContent* aCurrentContent,
+                             bool* aCheckSubDocument,
+                             nsIContent** aResultContent);
+
+  bool TryToMoveFocusToSubDocument(nsIContent* aCurrentContent,
+                                   nsIContent* aOriginalStartContent,
+                                   bool aForward,
+                                   bool aForDocumentNavigation,
+                                   nsIContent** aResultContent);
+
   // the currently active and front-most top-most window
   nsCOMPtr<nsPIDOMWindowOuter> mActiveWindow;