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

cc/trees/layer_tree_host_impl.h

Index: cc/trees/layer_tree_host_impl.h
diff --git a/cc/trees/layer_tree_host_impl.h b/cc/trees/layer_tree_host_impl.h
index 6b53b46c2d93cc0762504f59fb46855a80443b63..b14dd5a8b06b7a9947d67ee7636dd3ebd441f8cd 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -326,7 +326,8 @@ class CC_EXPORT LayerTreeHostImpl
 
   size_t SourceAnimationFrameNumberForTesting() const;
 
-  void RegisterScrollbarAnimationController(ElementId scroll_element_id);
+  void RegisterScrollbarAnimationController(ElementId scroll_element_id,
+                                            float initial_opacity);
   void UnregisterScrollbarAnimationController(ElementId scroll_element_id);
   ScrollbarAnimationController* ScrollbarAnimationControllerForElementId(
       ElementId scroll_element_id) const;
@@ -711,6 +712,10 @@ class CC_EXPORT LayerTreeHostImpl
   bool IsScrolledBy(LayerImpl* child, ScrollNode* ancestor);
   void ShowScrollbarsForImplScroll(ElementId element_id);
 
+  // Copy any opacity values already in the active tree to the pending
+  // tree, because the active tree value always takes precedence for scrollbars.
+  void PushScrollbarOpacitiesFromActiveToPending();
+
   using UIResourceMap = std::unordered_map<UIResourceId, UIResourceData>;
   UIResourceMap ui_resource_map_;