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

cc/input/single_scrollbar_animation_controller_thinning.h

 // Copyright 2013 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_INPUT_SINGLE_SCROLLBAR_ANIMATION_CONTROLLER_THINNING_H_
 #define CC_INPUT_SINGLE_SCROLLBAR_ANIMATION_CONTROLLER_THINNING_H_
 
 #include <memory>
 
 #include "base/macros.h"
 #include "base/time/time.h"
 #include "cc/cc_export.h"
 #include "cc/input/scrollbar.h"
 #include "cc/layers/layer_impl.h"
 #include "cc/layers/scrollbar_layer_impl_base.h"
 #include "ui/gfx/geometry/vector2d_f.h"
 
 namespace cc {
 
 class ScrollbarAnimationControllerClient;
 
 // ScrollbarAnimationControllerThinning for one scrollbar
 class CC_EXPORT SingleScrollbarAnimationControllerThinning {
  public:
   static constexpr float kIdleThicknessScale = 0.4f;
   static constexpr float kDefaultMouseMoveDistanceToTriggerAnimation = 25.f;
 
   static std::unique_ptr<SingleScrollbarAnimationControllerThinning> Create(
-      int scroll_layer_id,
+      ElementId scroll_element_id,
       ScrollbarOrientation orientation,
       ScrollbarAnimationControllerClient* client,
       base::TimeDelta thinning_duration);
 
   ~SingleScrollbarAnimationControllerThinning() {}
 
   bool mouse_is_over_scrollbar() const { return mouse_is_over_scrollbar_; }
   bool mouse_is_near_scrollbar() const { return mouse_is_near_scrollbar_; }
   bool captured() const { return captured_; }
 
   bool Animate(base::TimeTicks now);
   void StartAnimation();
   void StopAnimation();
 
   void UpdateThumbThicknessScale();
 
   void DidMouseDown();
   void DidMouseUp();
   void DidMouseLeave();
   void DidMouseMoveNear(float distance);
 
  private:
   SingleScrollbarAnimationControllerThinning(
-      int scroll_layer_id,
+      ElementId scroll_element_id,
       ScrollbarOrientation orientation,
       ScrollbarAnimationControllerClient* client,
       base::TimeDelta thinning_duration);
 
   float AnimationProgressAtTime(base::TimeTicks now);
   void RunAnimationFrame(float progress);
   const base::TimeDelta& Duration();
 
   // Describes whether the current animation should INCREASE (thicken)
   // a bar or DECREASE it (thin).
   enum AnimationChange { NONE, INCREASE, DECREASE };
   float ThumbThicknessScaleAt(float progress);
 
   float AdjustScale(float new_value,
                     float current_value,
                     AnimationChange animation_change,
                     float min_value,
                     float max_value);
   void ApplyThumbThicknessScale(float thumb_thickness_scale);
 
   ScrollbarAnimationControllerClient* client_;
 
   base::TimeTicks last_awaken_time_;
   bool is_animating_;
 
-  int scroll_layer_id_;
+  ElementId scroll_element_id_;
 
   ScrollbarOrientation orientation_;
   bool captured_;
   bool mouse_is_over_scrollbar_;
   bool mouse_is_near_scrollbar_;
   // Are we narrowing or thickening the bars.
   AnimationChange thickness_change_;
 
   base::TimeDelta thinning_duration_;
 
   DISALLOW_COPY_AND_ASSIGN(SingleScrollbarAnimationControllerThinning);
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_SINGLE_SCROLLBAR_ANIMATION_CONTROLLER_THINNING_H_