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

cc/layer.h

 // Copyright 2010 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_LAYER_H_
 #define CC_LAYER_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 33 matching lines @@
 class ScrollbarLayer;
 struct AnimationEvent;
 struct RenderingStats;
 
 // Base class for composited layers. Special layer types are derived from
 // this class.
 class CC_EXPORT Layer : public base::RefCounted<Layer>,
                         public LayerAnimationValueObserver {
  public:
   typedef std::vector<scoped_refptr<Layer> > LayerList;
+  enum LayerIdLabels {
+    INVALID_ID = -1,
+    NOTUSED_ID = 0

[enne (OOO), 2013/03/12 16:27:37]

Can you add a PINCH_ZOOM_ROOT_SCROLL_LAYER_ID = -2 and use that instead
explicitly? The name "NOTUSED" doesn't really correspond to what you're using
this for.

+  };
 
   static scoped_refptr<Layer> Create();
 
   int id() const { return layer_id_; }
 
   Layer* RootLayer();
   Layer* parent() { return parent_; }
   const Layer* parent() const { return parent_; }
   void AddChild(scoped_refptr<Layer> child);
   void InsertChild(scoped_refptr<Layer> child, size_t index);
@@ skipping 410 matching lines @@
   WebKit::WebLayerScrollClient* layer_scroll_client_;
 
   DrawProperties<Layer, RenderSurface> draw_properties_;
 
   DISALLOW_COPY_AND_ASSIGN(Layer);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYER_H_