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

cc/layer_impl.h

Index: cc/layer_impl.h
diff --git a/cc/layer_impl.h b/cc/layer_impl.h
index af96077efee758b206c681157fd5d67896e014b6..91de493b459b78fca9af573ef2d4245485810fd5 100644
--- a/cc/layer_impl.h
+++ b/cc/layer_impl.h
@@ -209,9 +209,12 @@ public:
     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&);
 
@@ -382,6 +385,7 @@ private:
     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