Mac: Don't repaint scrollbars every frame

cc/layers/painted_scrollbar_layer_impl.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_LAYERS_PAINTED_SCROLLBAR_LAYER_IMPL_H_
 #define CC_LAYERS_PAINTED_SCROLLBAR_LAYER_IMPL_H_
 
 #include "cc/base/cc_export.h"
 #include "cc/input/scrollbar.h"
 #include "cc/layers/scrollbar_layer_impl_base.h"
@@ skipping 27 matching lines @@
   void SetTrackStart(int track_start);
   void SetTrackLength(int track_length);
 
   void set_track_ui_resource_id(UIResourceId uid) {
     track_ui_resource_id_ = uid;
   }
   void set_thumb_ui_resource_id(UIResourceId uid) {
     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;
     internal_content_bounds_ = content_bounds;
   }
 
  protected:
   PaintedScrollbarLayerImpl(LayerTreeImpl* tree_impl,
                             int id,
                             ScrollbarOrientation orientation);
 
   // ScrollbarLayerImplBase implementation.
   int ThumbThickness() const override;
   int ThumbLength() const override;
   float TrackLength() const override;
   int TrackStart() const override;
   bool IsThumbResizable() const override;
 
  private:
   const char* LayerTypeAsString() const override;
 
   UIResourceId track_ui_resource_id_;
   UIResourceId thumb_ui_resource_id_;
 
+  float thumb_opacity_;
+
   float internal_contents_scale_;
   gfx::Size internal_content_bounds_;
 
   int thumb_thickness_;
   int thumb_length_;
   int track_start_;
   int track_length_;
 
   DISALLOW_COPY_AND_ASSIGN(PaintedScrollbarLayerImpl);
 };
 
 }  // namespace cc
 #endif  // CC_LAYERS_PAINTED_SCROLLBAR_LAYER_IMPL_H_