Replace layer id with Element id for tracking scrollbar animation controllers

cc/layers/scrollbar_layer_impl_base.cc

 // 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.
 
 #include "cc/layers/scrollbar_layer_impl_base.h"
 
 #include <algorithm>
 #include "cc/trees/effect_node.h"
 #include "cc/trees/layer_tree_impl.h"
 #include "ui/gfx/geometry/rect_conversions.h"
@@ skipping 18 matching lines @@
       vertical_adjust_(0.f) {}
 
 ScrollbarLayerImplBase::~ScrollbarLayerImplBase() {
   layer_tree_impl()->UnregisterScrollbar(this);
 }
 
 void ScrollbarLayerImplBase::PushPropertiesTo(LayerImpl* layer) {
   LayerImpl::PushPropertiesTo(layer);
   DCHECK(layer->ToScrollbarLayer());
   layer->ToScrollbarLayer()->set_is_overlay_scrollbar(is_overlay_scrollbar_);
-  layer->ToScrollbarLayer()->SetScrollLayerId(ScrollLayerId());
+  layer->ToScrollbarLayer()->SetScrollInfo(ScrollLayerId(),
+                                           scroll_element_id());
 }
 
 ScrollbarLayerImplBase* ScrollbarLayerImplBase::ToScrollbarLayer() {
   return this;
 }
 
-void ScrollbarLayerImplBase::SetScrollLayerId(int scroll_layer_id) {
-  if (scroll_layer_id_ == scroll_layer_id)
+void ScrollbarLayerImplBase::SetScrollInfo(int scroll_layer_id,
+                                           ElementId scroll_element_id) {
+  if (scroll_layer_id_ == scroll_layer_id &&
+      scroll_element_id == scroll_element_id)

[pdr., 2017/04/20 02:53:42]

Oops, "scroll_element_id == scroll_element_id" instead of "scroll_element_id_ ==
scroll_element_id". Fixing in followup:
https://codereview.chromium.org/2827163005

     return;
 
   layer_tree_impl()->UnregisterScrollbar(this);
 
   scroll_layer_id_ = scroll_layer_id;
+  scroll_element_id_ = scroll_element_id;
 
   layer_tree_impl()->RegisterScrollbar(this);
 }
 
 bool ScrollbarLayerImplBase::SetCurrentPos(float current_pos) {
   if (current_pos_ == current_pos)
     return false;
   current_pos_ = current_pos;
   NoteLayerPropertyChanged();
   return true;
@@ skipping 163 matching lines @@
   layer_tree_impl()->AddToOpacityAnimationsMap(id(), opacity);
 
   node->opacity = opacity;
   node->effect_changed = true;
   property_trees->changed = true;
   property_trees->effect_tree.set_needs_update(true);
   layer_tree_impl()->set_needs_update_draw_properties();
 }
 
 }  // namespace cc