Mac: Don't repaint scrollbars every frame

cc/layers/painted_scrollbar_layer_impl.h

Index: cc/layers/painted_scrollbar_layer_impl.h
diff --git a/cc/layers/painted_scrollbar_layer_impl.h b/cc/layers/painted_scrollbar_layer_impl.h
index 65d28aea0501f0b172ea1497763e2b7074b460a6..190526f78aa4d46ef28cf491a3f46c3aef58c91d 100644
--- a/cc/layers/painted_scrollbar_layer_impl.h
+++ b/cc/layers/painted_scrollbar_layer_impl.h
@@ -45,6 +45,8 @@ class CC_EXPORT PaintedScrollbarLayerImpl : public ScrollbarLayerImplBase {
     thumb_ui_resource_id_ = uid;
   }
 
+  void set_thumb_opacity(float opacity) { thumb_opacity_ = opacity; }
+
   void set_internal_contents_scale_and_bounds(float content_scale,
                                               const gfx::Size& content_bounds) {
     internal_contents_scale_ = content_scale;
@@ -69,6 +71,8 @@ class CC_EXPORT PaintedScrollbarLayerImpl : public ScrollbarLayerImplBase {
   UIResourceId track_ui_resource_id_;
   UIResourceId thumb_ui_resource_id_;
 
+  float thumb_opacity_;
+
   float internal_contents_scale_;
   gfx::Size internal_content_bounds_;