Replace layer id with Element id for tracking scrollbar animation controllers

cc/layers/scrollbar_layer_impl_base.cc

Index: cc/layers/scrollbar_layer_impl_base.cc
diff --git a/cc/layers/scrollbar_layer_impl_base.cc b/cc/layers/scrollbar_layer_impl_base.cc
index da53036fea0a07cd555c932d769ce32092f6aa19..e9c5514d5f9022fd7c57c9466f7f7d2ba449a586 100644
--- a/cc/layers/scrollbar_layer_impl_base.cc
+++ b/cc/layers/scrollbar_layer_impl_base.cc
@@ -36,20 +36,24 @@ 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);
 }