Implement one-page-at-a-time mousewheel scrolling in the impl thread.

cc/layer_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_IMPL_H_
 #define CC_LAYER_IMPL_H_
 
 #include <string>
 
 #include "base/logging.h"
@@ skipping 191 matching lines @@
 
     virtual void calculateContentsScale(
         float idealContentsScale,
         float* contentsScaleX,
         float* contentsScaleY,
         gfx::Size* contentBounds);
 
     gfx::Vector2d scrollOffset() const { return m_scrollOffset; }
     void setScrollOffset(gfx::Vector2d);
 
-    gfx::Vector2d maxScrollOffset() const {return m_maxScrollOffset; }
+    gfx::Vector2d maxScrollOffset() const { return m_maxScrollOffset; }
     void setMaxScrollOffset(gfx::Vector2d);
 
+    gfx::Size visibleScrollArea() const { return m_visibleScrollArea; }
+    void setVisibleScrollArea(gfx::Size);
+
     const gfx::Vector2dF& scrollDelta() const { return m_scrollDelta; }
     void setScrollDelta(const gfx::Vector2dF&);
 
     const gfx::Transform& implTransform() const { return m_implTransform; }
     void setImplTransform(const gfx::Transform& transform);
 
     const gfx::Vector2d& sentScrollDelta() const { return m_sentScrollDelta; }
     void setSentScrollDelta(const gfx::Vector2d& sentScrollDelta);
 
     // Returns the delta of the scroll that was outside of the bounds of the initial scroll
@@ skipping 150 matching lines @@
     bool m_forceRenderSurface;
 
     // Set for the layer that other layers are fixed to.
     bool m_isContainerForFixedPositionLayers;
     // This is true if the layer should be fixed to the closest ancestor container.
     bool m_fixedToContainerLayer;
 
     gfx::Vector2dF m_scrollDelta;
     gfx::Vector2d m_sentScrollDelta;
     gfx::Vector2d m_maxScrollOffset;
+    gfx::Size m_visibleScrollArea;
     gfx::Transform m_implTransform;
 
     // The global depth value of the center of the layer. This value is used
     // to sort layers from back to front.
     float m_drawDepth;
 
     // Debug layer name.
     std::string m_debugName;
 
     WebKit::WebFilterOperations m_filters;
@@ skipping 18 matching lines @@
     // Group of properties that need to be computed based on the layer tree
     // hierarchy before layers can be drawn.
     DrawProperties<LayerImpl, RenderSurfaceImpl> m_drawProperties;
 
     DISALLOW_COPY_AND_ASSIGN(LayerImpl);
 };
 
 }
 
 #endif  // CC_LAYER_IMPL_H_