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

cc/layer_tree_host_impl.h

 // Copyright 2011 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_TREE_HOST_IMPL_H_
 #define CC_LAYER_TREE_HOST_IMPL_H_
 
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/time.h"
@@ skipping 224 matching lines @@
         size_t renderPassListBegin(const RenderPassList&) const { return 0; }
         size_t renderPassListEnd(const RenderPassList& list) const { return list.size(); }
         size_t renderPassListNext(size_t it) const { return it + 1; }
     };
 
     template<typename RenderPassCuller>
     static void removeRenderPasses(RenderPassCuller, FrameData&);
 
     float totalPageScaleFactorForTesting() const { return m_pinchZoomViewport.total_page_scale_factor(); }
 
-    const PinchZoomViewport& pinchZoomViewport() const { return m_pinchZoomViewport; }
+    PinchZoomViewport& pinchZoomViewport() { return m_pinchZoomViewport; }
 
     skia::RefPtr<SkPicture> capturePicture();
 
     bool pinchGestureActive() const { return m_pinchGestureActive; }
 
 protected:
     LayerTreeHostImpl(const LayerTreeSettings&, LayerTreeHostImplClient*, Proxy*);
     void activatePendingTree();
 
     // Virtual for testing.
@@ skipping 84 matching lines @@
     size_t m_lastSentMemoryUseBytes;
 
     scoped_ptr<AnimationRegistrar> m_animationRegistrar;
 
     DISALLOW_COPY_AND_ASSIGN(LayerTreeHostImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYER_TREE_HOST_IMPL_H_