Replace layer id with Element id for tracking scrollbar animation controllers

cc/layers/painted_overlay_scrollbar_layer.h

Index: cc/layers/painted_overlay_scrollbar_layer.h
diff --git a/cc/layers/painted_overlay_scrollbar_layer.h b/cc/layers/painted_overlay_scrollbar_layer.h
index f7cd66e5fd52a0ada74f33939581e815d45c6f1d..a13029c5354e51f0395999ae58b215998685e78c 100644
--- a/cc/layers/painted_overlay_scrollbar_layer.h
+++ b/cc/layers/painted_overlay_scrollbar_layer.h
@@ -22,7 +22,8 @@ class CC_EXPORT PaintedOverlayScrollbarLayer : public ScrollbarLayerInterface,
 
   static scoped_refptr<PaintedOverlayScrollbarLayer> Create(
       std::unique_ptr<Scrollbar> scrollbar,
-      int scroll_layer_id);
+      int scroll_layer_id,
+      ElementId scroll_element_id = ElementId());
 
   bool OpacityCanAnimateOnImplThread() const override;
   bool AlwaysUseActiveTreeOpacity() const override;
@@ -30,7 +31,8 @@ class CC_EXPORT PaintedOverlayScrollbarLayer : public ScrollbarLayerInterface,
 
   // ScrollbarLayerInterface
   int ScrollLayerId() const override;
-  void SetScrollLayer(int layer_id) override;
+  ElementId scroll_element_id() const override;
+  void SetScrollInfo(int layer_id, ElementId element_id) override;

[enne (OOO), 2017/04/18 00:47:57]

Can you leave a TODO that layer id is deprecated here?

Is there a reason to not just remove it in this patch?

[pdr., 2017/04/18 04:34:11]

Done, I added a comment here and to all other ScrollbarLayerInterface clients
that scrollbars should be refactored to just rely on element ids.
The scroll layer -> scrollbar layer map still depends on these layer ids (e.g.,
LayerTreeImpl::RegisterScrollbar). In a followup
(https://codereview.chromium.org/2824693002), I will refactor the scrollbar
mapping to use element ids and then we'll be able to switch SetScrollInfo to
just be SetScrollElementId.

In writing this response, I realized we can remove the ScrollLayerId accessor
from ScrollbarLayerInterface but we still need to pass it in to SetScrollInfo
for now.

   ScrollbarOrientation orientation() const override;
 
   // Layer interface
@@ -40,7 +42,8 @@ class CC_EXPORT PaintedOverlayScrollbarLayer : public ScrollbarLayerInterface,
 
  protected:
   PaintedOverlayScrollbarLayer(std::unique_ptr<Scrollbar> scrollbar,
-                               int scroll_layer_id);
+                               int scroll_layer_id,
+                               ElementId scroll_element_id);
   ~PaintedOverlayScrollbarLayer() override;
 
  private:
@@ -59,6 +62,7 @@ class CC_EXPORT PaintedOverlayScrollbarLayer : public ScrollbarLayerInterface,
 
   std::unique_ptr<Scrollbar> scrollbar_;
   int scroll_layer_id_;
+  ElementId scroll_element_id_;
 
   int thumb_thickness_;
   int thumb_length_;