cc : Initialize solid color scrollbar layer on main thread to 0.

cc/layers/layer.h

Index: cc/layers/layer.h
diff --git a/cc/layers/layer.h b/cc/layers/layer.h
index 47ed62243490f4d44b56b6e833b172c0273c3ea4..78cf2fa10ba10ee13a9689ba35682302b8d6f62e 100644
--- a/cc/layers/layer.h
+++ b/cc/layers/layer.h
@@ -128,7 +128,7 @@ class CC_EXPORT Layer : public base::RefCounted<Layer> {
   virtual void SetNeedsDisplayRect(const gfx::Rect& dirty_rect);
   void SetNeedsDisplay() { SetNeedsDisplayRect(gfx::Rect(bounds())); }
 
-  void SetOpacity(float opacity);
+  virtual void SetOpacity(float opacity);
   float opacity() const { return opacity_; }
   float EffectiveOpacity() const;
   bool OpacityIsAnimating() const;