Do not invalidate more children than necessary

Do not invalidate children all the way up to the unignored ancestor,
only invalidate a single node included in tree, either |this| or the
first ancestor included in the tree.

This is a precursor to fixing terrible performance loading large chat
rooms on chat.google.com, but it does not yet fix the issue nor
the related perf_tests. In order to fix that, it will be necessary
to include more nodes in the tree as well.

Bug: 1107988
Change-Id: I8a1f267a243ff1eda3d186e4c872eac75b40d792
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2383293
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803151}

third_party/blink/renderer/modules/accessibility/ax_node_object.cc

@@ -3740,13 +3740,11 @@ void AXNodeObject::ChildrenChanged() {
   // because unignored nodes recursively include all children of ignored
   // nodes. This method is called during layout, so we need to be careful to
   // only explore existing objects.
-  AXObject* node_to_update = this;
-  while (node_to_update) {
+  bool is_included =
+      !LastKnownIsIgnoredValue() || LastKnownIsIgnoredButIncludedInTreeValue();
+  AXObject* node_to_update = is_included ? this : ParentObjectIncludedInTree();
+  if (node_to_update)  // Can be null, e.g. if <title> contents change.
     node_to_update->SetNeedsToUpdateChildren();
-    if (!node_to_update->LastKnownIsIgnoredValue())
-      break;
-    node_to_update = node_to_update->ParentObjectIfExists();
-  }
 
   // If this node's children are not part of the accessibility tree then
   // skip notification and walking up the ancestors.