Implement pinch-zoom scaling for main-frame scrollbars and pinch-zoom overlay scrollbars.

cc/scrollbar_layer.h

 // Copyright 2012 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_SCROLLBAR_LAYER_H_
 #define CC_SCROLLBAR_LAYER_H_
 
 #include "cc/cc_export.h"
 #include "cc/contents_scaling_layer.h"
 #include "cc/layer_updater.h"
@@ skipping 14 matching lines @@
 
   static scoped_refptr<ScrollbarLayer> Create(
       scoped_ptr<WebKit::WebScrollbar>,
       scoped_ptr<ScrollbarThemePainter>,
       scoped_ptr<WebKit::WebScrollbarThemeGeometry>,
       int scrollLayerId);
 
   int scroll_layer_id() const { return scroll_layer_id_; }
   void SetScrollLayerId(int id);
 
+  virtual bool DoNotSkipOnZeroOpacity() const OVERRIDE;
+
   WebKit::WebScrollbar::Orientation Orientation() const;
 
   // Layer interface
   virtual void SetTexturePriorities(const PriorityCalculator& priority_calc)
       OVERRIDE;
   virtual void Update(ResourceUpdateQueue* queue,
                       const OcclusionTracker* occlusion,
                       RenderingStats* stats) OVERRIDE;
   virtual void SetLayerTreeHost(LayerTreeHost* host) OVERRIDE;
   virtual void PushPropertiesTo(LayerImpl* layer) OVERRIDE;
@@ skipping 43 matching lines @@
 
   // All the parts of the scrollbar except the thumb
   scoped_ptr<LayerUpdater::Resource> back_track_;
   scoped_ptr<LayerUpdater::Resource> fore_track_;
   scoped_ptr<LayerUpdater::Resource> thumb_;
 };
 
 }  // namespace cc
 
 #endif  // CC_SCROLLBAR_LAYER_H_