Bug 1321284 - Crash in nsCSSFrameConstructor::GetInsertionPrevSibling when trying to reframe native anonymous content

* Make StyleChildrenIterator skip NAC generated by root element primary frame ancestors.
* Add nsINode::GetFlattenedTreeParentNodeForStyle.
* Add iterator class to find all restyle roots.

NOTE: Parts 1, 2, and "4.2"

Tag #1375

layout/style/DocumentStyleRootIterator.cpp

@@ -0,0 +1,41 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "DocumentStyleRootIterator.h"
+
+#include "nsContentUtils.h"
+
+namespace mozilla {
+
+DocumentStyleRootIterator::DocumentStyleRootIterator(nsIDocument* aDocument)
+  : mPosition(0)
+{
+  MOZ_COUNT_CTOR(DocumentStyleRootIterator);
+  if (Element* root = aDocument->GetRootElement()) {
+    mStyleRoots.AppendElement(root);
+  }
+  nsContentUtils::AppendDocumentLevelNativeAnonymousContentTo(
+      aDocument, mStyleRoots);
+}
+
+Element*
+DocumentStyleRootIterator::GetNextStyleRoot()
+{
+  for (;;) {
+    if (mPosition >= mStyleRoots.Length()) {
+      return nullptr;
+    }
+
+    nsIContent* next = mStyleRoots[mPosition];
+    ++mPosition;
+
+    if (next->IsElement()) {
+      return next->AsElement();
+    }
+  }
+}
+
+} // namespace mozilla