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

cc/trees/layer_tree_host_impl.h

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_TREES_LAYER_TREE_HOST_IMPL_H_
 #define CC_TREES_LAYER_TREE_HOST_IMPL_H_
 
 #include <stddef.h>
 
 #include <bitset>
@@ skipping 308 matching lines @@
 
   // Prevents notifying the |client_| when an impl side invalidation request is
   // made. When unblocked, the disabled request will immediately be called.
   virtual void BlockImplSideInvalidationRequestsForTesting(bool block);
 
   // Resets all of the trees to an empty state.
   void ResetTreesForTesting();
 
   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;
 
   DrawMode GetDrawMode() const;
 
   // TileManagerClient implementation.
   void NotifyReadyToActivate() override;
   void NotifyReadyToDraw() override;
   void NotifyAllTileTasksCompleted() override;
@@ skipping 364 matching lines @@
   // compositor side. The information gets shared by the main thread as part of
   // the begin_main_frame_state. Finally Use counters are updated in the main
   // thread side to keep track of the frequency of scrolling with different
   // sources per page load. TODO(crbug.com/691886): Use GRC API to plumb the
   // scroll source info for Use Counters.
   void UpdateScrollSourceInfo(bool is_wheel_scroll);
 
   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_;
 
   // Resources that were evicted by EvictAllUIResources. Resources are removed
   // from this when they are touched by a create or destroy from the UI resource
   // request queue.
   std::set<UIResourceId> evicted_ui_resources_;
 
   CompositorFrameSink* compositor_frame_sink_;
 
@@ skipping 148 matching lines @@
   bool touchpad_and_wheel_scroll_latching_enabled_;
 
   ImplThreadPhase impl_thread_phase_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHostImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_IMPL_H_