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

cc/layer.h

Index: cc/layer.h
diff --git a/cc/layer.h b/cc/layer.h
index 5f61d1428847ee1ce5966b0d8a42a6d71f12d9b2..f1aca620f06151d7e7c4df6ccd8a9523d57a85a1 100644
--- a/cc/layer.h
+++ b/cc/layer.h
@@ -155,6 +155,9 @@ public:
     void setMaxScrollOffset(gfx::Vector2d);
     gfx::Vector2d maxScrollOffset() const { return m_maxScrollOffset; }
 
+    void setVisibleScrollArea(gfx::Size);
+    gfx::Size visibleScrollArea() const { return m_visibleScrollArea; }
+
     void setScrollable(bool);
     bool scrollable() const { return m_scrollable; }
 
@@ -358,6 +361,7 @@ private:
 
     gfx::Vector2d m_scrollOffset;
     gfx::Vector2d m_maxScrollOffset;
+    gfx::Size m_visibleScrollArea;
     bool m_scrollable;
     bool m_shouldScrollOnMainThread;
     bool m_haveWheelEventHandlers;