Optimize BlinkAXTreeSource by adding freeze/thaw for things like root, focus.

content/renderer/accessibility/render_accessibility_impl.cc

Index: content/renderer/accessibility/render_accessibility_impl.cc
diff --git a/content/renderer/accessibility/render_accessibility_impl.cc b/content/renderer/accessibility/render_accessibility_impl.cc
index 4538ca414641193148ef0f2e344fe200a103f018..8720aa145470ef41f3053efcb3f46769130c3003 100644
--- a/content/renderer/accessibility/render_accessibility_impl.cc
+++ b/content/renderer/accessibility/render_accessibility_impl.cc
@@ -58,6 +58,7 @@ void RenderAccessibilityImpl::SnapshotAccessibilityTree(
   WebScopedAXContext context(document);
   BlinkAXTreeSource tree_source(render_frame);
   tree_source.SetRoot(context.root());
+  ScopedFreezeBlinkAXTreeSource freeze(&tree_source);
   BlinkAXTreeSerializer serializer(&tree_source);
   serializer.set_max_node_count(kMaxSnapshotNodeCount);
   serializer.SerializeChanges(context.root(), response);
@@ -244,6 +245,7 @@ void RenderAccessibilityImpl::SetPdfTreeSource(
   pdf_tree_source_ = pdf_tree_source;
   pdf_serializer_.reset(new PdfAXTreeSerializer(pdf_tree_source_));
 
+  ScopedFreezeBlinkAXTreeSource freeze(&tree_source_);
   WebAXObject root = tree_source_.GetRoot();
   if (!root.updateLayoutAndCheckValidity())
     return;
@@ -315,6 +317,8 @@ void RenderAccessibilityImpl::SendPendingAccessibilityEvents() {
     while (!obj.isDetached() && obj.accessibilityIsIgnored())
       obj = obj.parentObject();
 
+    ScopedFreezeBlinkAXTreeSource freeze(&tree_source_);
+
     // Make sure it's a descendant of our root node - exceptions include the
     // scroll area that's the parent of the main document (we ignore it), and
     // possibly nodes attached to a different document.
@@ -357,6 +361,7 @@ void RenderAccessibilityImpl::SendLocationChanges() {
   std::vector<AccessibilityHostMsg_LocationChangeParams> messages;
 
   // Update layout on the root of the tree.
+  ScopedFreezeBlinkAXTreeSource freeze(&tree_source_);
   WebAXObject root = tree_source_.GetRoot();
   if (!root.updateLayoutAndCheckValidity())
     return;
@@ -441,6 +446,7 @@ void RenderAccessibilityImpl::OnHitTest(gfx::Point point) {
   // message back to the browser to do the hit test in the child frame,
   // recursively.
   AXContentNodeData data;
+  ScopedFreezeBlinkAXTreeSource freeze(&tree_source_);
   tree_source_.SerializeNode(obj, &data);
   if (data.HasContentIntAttribute(AX_CONTENT_ATTR_CHILD_ROUTING_ID) ||
       data.HasContentIntAttribute(
@@ -455,6 +461,7 @@ void RenderAccessibilityImpl::OnHitTest(gfx::Point point) {
 }
 
 void RenderAccessibilityImpl::OnSetAccessibilityFocus(int acc_obj_id) {
+  ScopedFreezeBlinkAXTreeSource freeze(&tree_source_);
   if (tree_source_.accessibility_focus_id() == acc_obj_id)
     return;