Fix cc scrollbar layer issues with initialization, and use element ids throughout.

cc/trees/property_tree.h

Index: cc/trees/property_tree.h
diff --git a/cc/trees/property_tree.h b/cc/trees/property_tree.h
index 601cbaf2f74d77f52e744ff0a075d93fd59b904c..3ea602ad1c09a095b48ef58f627b74ba8ae24282 100644
--- a/cc/trees/property_tree.h
+++ b/cc/trees/property_tree.h
@@ -647,7 +647,6 @@ class CC_EXPORT PropertyTrees final {
   base::flat_map<ElementId, int> element_id_to_scroll_node_index;
   base::flat_map<ElementId, int> element_id_to_transform_node_index;
 
-  std::vector<int> always_use_active_tree_opacity_effect_ids;
   TransformTree transform_tree;
   EffectTree effect_tree;
   ClipTree clip_tree;
@@ -674,7 +673,6 @@ class CC_EXPORT PropertyTrees final {
   void SetInnerViewportContainerBoundsDelta(gfx::Vector2dF bounds_delta);
   void SetOuterViewportContainerBoundsDelta(gfx::Vector2dF bounds_delta);
   void SetInnerViewportScrollBoundsDelta(gfx::Vector2dF bounds_delta);
-  void PushOpacityIfNeeded(PropertyTrees* target_tree);
   void RemoveIdFromIdToIndexMaps(int id);
   void UpdateChangeTracking();
   void PushChangeTrackingTo(PropertyTrees* tree);