Refactor LayerTreeImpl's scrollbar map to be keyed on element ids

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 304 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(int scroll_layer_id,
-                                            ElementId scroll_element_id);
+  void RegisterScrollbarAnimationController(ElementId scroll_element_id);
   void UnregisterScrollbarAnimationController(ElementId scroll_element_id);
   ScrollbarAnimationController* ScrollbarAnimationControllerForElementId(
       ElementId scroll_element_id) const;
 
   DrawMode GetDrawMode() const;
 
   // Viewport size in draw space: this size is in physical pixels and is used
   // for draw properties, tilings, quads and render passes.
   gfx::Size DrawViewportSize() const;
 
@@ skipping 12 matching lines @@
       TreePriority tree_priority) override;
   void SetIsLikelyToRequireADraw(bool is_likely_to_require_a_draw) override;
   gfx::ColorSpace GetRasterColorSpace() const override;
   void RequestImplSideInvalidation() override;
 
   // ScrollbarAnimationControllerClient implementation.
   void PostDelayedScrollbarAnimationTask(const base::Closure& task,
                                          base::TimeDelta delay) override;
   void SetNeedsAnimateForScrollbarAnimation() override;
   void SetNeedsRedrawForScrollbarAnimation() override;
-  ScrollbarSet ScrollbarsFor(int scroll_layer_id) const override;
+  ScrollbarSet ScrollbarsFor(ElementId scroll_element_id) const override;
   void DidChangeScrollbarVisibility() override;
 
   // VideoBeginFrameSource implementation.
   void AddVideoFrameController(VideoFrameController* controller) override;
   void RemoveVideoFrameController(VideoFrameController* controller) override;
 
   // CompositorFrameSinkClient implementation.
   void SetBeginFrameSource(BeginFrameSource* source) override;
   void SetExternalTilePriorityConstraints(
       const gfx::Rect& viewport_rect,
@@ skipping 504 matching lines @@
   bool has_scrolled_by_touch_;
 
   bool touchpad_and_wheel_scroll_latching_enabled_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHostImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_IMPL_H_