Bug 1409975 - Implement node distribution for shadow tree slots

* Implementation for assignedNodes
* Include slots in the flat tree
* Fix event get-the-parent algorithm for a node
* Update and add reftests for Shadow DOM v1
* Update web platform tests expectations

Tag #1375

dom/events/EventDispatcher.h

@@ -204,13 +204,6 @@ public:
    * which should be used when the event is handled at mParentTarget.
    */
   dom::EventTarget* mEventTargetAtParent;
-
-  /**
-   * An array of destination insertion points that need to be inserted
-   * into the event path of nodes that are distributed by the
-   * web components distribution algorithm.
-   */
-  nsTArray<nsIContent*> mDestInsertionPoints;
 };
 
 class EventChainPostVisitor : public mozilla::EventChainVisitor