Replace layer id with Element id for tracking scrollbar animation controllers

cc/layers/painted_scrollbar_layer.h

Index: cc/layers/painted_scrollbar_layer.h
diff --git a/cc/layers/painted_scrollbar_layer.h b/cc/layers/painted_scrollbar_layer.h
index 8e1884f44f53671118101b62359ebcd26f49cfb6..28d50e443e16e51906685bec929f6c47f960da3b 100644
--- a/cc/layers/painted_scrollbar_layer.h
+++ b/cc/layers/painted_scrollbar_layer.h
@@ -22,7 +22,8 @@ class CC_EXPORT PaintedScrollbarLayer : public ScrollbarLayerInterface,
 
   static scoped_refptr<PaintedScrollbarLayer> Create(
       std::unique_ptr<Scrollbar> scrollbar,
-      int scroll_layer_id);
+      int scroll_layer_id,
+      ElementId element_id);

[wkorman, 2017/04/14 18:17:30]

We could add a default '= ElementId()' here and then tests and other places that
are just passing default instance wouldn't need to be changed. I see some
precedent in cc code, see for example cc/paint/paint_flags.h getFillPath.

[pdr., 2017/04/15 06:02:00]

This saves many lines in the diff. Done

 
   bool OpacityCanAnimateOnImplThread() const override;
   bool AlwaysUseActiveTreeOpacity() const override;
@@ -30,7 +31,8 @@ class CC_EXPORT PaintedScrollbarLayer : public ScrollbarLayerInterface,
 
   // ScrollbarLayerInterface
   int ScrollLayerId() const override;
-  void SetScrollLayer(int layer_id) override;
+  ElementId ScrollElementId() const override;
+  void SetScrollInfo(int layer_id, ElementId element_id) override;
 
   ScrollbarOrientation orientation() const override;
 
@@ -45,7 +47,8 @@ class CC_EXPORT PaintedScrollbarLayer : public ScrollbarLayerInterface,
 
  protected:
   PaintedScrollbarLayer(std::unique_ptr<Scrollbar> scrollbar,
-                        int scroll_layer_id);
+                        int scroll_layer_id,
+                        ElementId scroll_element_id);
   ~PaintedScrollbarLayer() override;
 
   // For unit tests
@@ -77,6 +80,7 @@ class CC_EXPORT PaintedScrollbarLayer : public ScrollbarLayerInterface,
 
   std::unique_ptr<Scrollbar> scrollbar_;
   int scroll_layer_id_;
+  ElementId scroll_element_id_;
 
   float internal_contents_scale_;
   gfx::Size internal_content_bounds_;