Mac: Don't repaint scrollbars every frame

cc/blink/scrollbar_impl.h

 // Copyright 2014 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_BLINK_SCROLLBAR_IMPL_H_
 #define CC_BLINK_SCROLLBAR_IMPL_H_
 
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
 #include "cc/input/scrollbar.h"
@@ skipping 15 matching lines @@
 
   // cc::Scrollbar implementation.
   cc::ScrollbarOrientation Orientation() const override;
   bool IsLeftSideVerticalScrollbar() const override;
   bool HasThumb() const override;
   bool IsOverlay() const override;
   gfx::Point Location() const override;
   int ThumbThickness() const override;
   int ThumbLength() const override;
   gfx::Rect TrackRect() const override;
+  float ThumbOpacity() const override;
+  float TrackOpacity() const override;
+  bool NeedsPaintPart(cc::ScrollbarPart part) const override;
   void PaintPart(SkCanvas* canvas,
                  cc::ScrollbarPart part,
                  const gfx::Rect& content_rect) override;
 
  private:
   scoped_ptr<blink::WebScrollbar> scrollbar_;
   blink::WebScrollbarThemePainter painter_;
   scoped_ptr<blink::WebScrollbarThemeGeometry> geometry_;
 
   DISALLOW_COPY_AND_ASSIGN(ScrollbarImpl);
 };
 
 }  // namespace cc_blink
 
 #endif  // CC_BLINK_SCROLLBAR_IMPL_H_