Replace layer id with Element id for tracking scrollbar animation controllers

cc/test/fake_painted_scrollbar_layer.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_TEST_FAKE_PAINTED_SCROLLBAR_LAYER_H_
 #define CC_TEST_FAKE_PAINTED_SCROLLBAR_LAYER_H_
 
 #include <stddef.h>
 
 #include <memory>
 
 #include "cc/layers/painted_scrollbar_layer.h"
 #include "cc/test/fake_scrollbar.h"
 
 namespace base { template<typename T> class AutoReset; }
 
 namespace cc {
 
 class FakePaintedScrollbarLayer : public PaintedScrollbarLayer {
  public:
   static scoped_refptr<FakePaintedScrollbarLayer> Create(
       bool paint_during_update,
       bool has_thumb,
-      int scrolling_layer_id);
+      int scrolling_layer_id,
+      ElementId scrolling_element_id = ElementId());
   static scoped_refptr<FakePaintedScrollbarLayer> Create(
       bool paint_during_update,
       bool has_thumb,
       ScrollbarOrientation orientation,
       bool is_left_side_vertical_scrollbar,
       bool is_overlay,
-      int scrolling_layer_id);
+      int scrolling_layer_id,
+      ElementId scrolling_element_id = ElementId());
   int update_count() const { return update_count_; }
   void reset_update_count() { update_count_ = 0; }
 
   bool Update() override;
 
   void PushPropertiesTo(LayerImpl* layer) override;
 
   std::unique_ptr<base::AutoReset<bool>> IgnoreSetNeedsCommit();
 
   size_t push_properties_count() const { return push_properties_count_; }
   void reset_push_properties_count() { push_properties_count_ = 0; }
 
   // For unit tests
   UIResourceId track_resource_id() {
     return PaintedScrollbarLayer::track_resource_id();
   }
   UIResourceId thumb_resource_id() {
     return PaintedScrollbarLayer::thumb_resource_id();
   }
   FakeScrollbar* fake_scrollbar() {
     return fake_scrollbar_;
   }
   using PaintedScrollbarLayer::UpdateInternalContentScale;
   using PaintedScrollbarLayer::UpdateThumbAndTrackGeometry;
 
  private:
   FakePaintedScrollbarLayer(FakeScrollbar* fake_scrollbar,
-                            int scrolling_layer_id);
+                            int scrolling_layer_id,
+                            ElementId scrolling_element_id);
   ~FakePaintedScrollbarLayer() override;
 
   int update_count_;
   size_t push_properties_count_;
   FakeScrollbar* fake_scrollbar_;
 };
 
 }  // namespace cc
 
 #endif  // CC_TEST_FAKE_PAINTED_SCROLLBAR_LAYER_H_