Refactor cc scrollbar layers to separate solid-color vs desktop.

cc/layers/layer.h

Index: cc/layers/layer.h
diff --git a/cc/layers/layer.h b/cc/layers/layer.h
index 7a8f9d1532b2721f8f01e830a0d9de00bdfd1cdb..737e83df0671bb0a73883ebec880df5960276cf4 100644
--- a/cc/layers/layer.h
+++ b/cc/layers/layer.h
@@ -48,10 +48,10 @@ class LayerClient;
 class LayerImpl;
 class LayerTreeHost;
 class LayerTreeImpl;
-class PaintedScrollbarLayer;
 class PriorityCalculator;
 class RenderingStatsInstrumentation;
 class ResourceUpdateQueue;
+class ScrollbarLayerInterface;
 struct AnimationEvent;
 
 // Base class for composited layers. Special layer types are derived from
@@ -393,7 +393,7 @@ class CC_EXPORT Layer : public base::RefCounted<Layer>,
 
   virtual Region VisibleContentOpaqueRegion() const;
 
-  virtual PaintedScrollbarLayer* ToScrollbarLayer();
+  virtual ScrollbarLayerInterface* ToScrollbarLayer();
 
   gfx::Rect LayerRectToContentRect(const gfx::RectF& layer_rect) const;