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

cc/input_handler.h

Index: cc/input_handler.h
diff --git a/cc/input_handler.h b/cc/input_handler.h
index 2016f282c75c887f9e49bc68cc50fc7358e5ceab..66cfabbad91e090ae5adf9f1149e623d07a912f1 100644
--- a/cc/input_handler.h
+++ b/cc/input_handler.h
@@ -46,7 +46,9 @@ public:
     // direction at all, then false is returned. If any layer is moved, then
     // true is returned.
     // Should only be called if scrollBegin() returned ScrollStarted.
-    virtual bool scrollBy(const gfx::Point&, const gfx::Vector2d&) = 0;
+    virtual bool scrollBy(const gfx::Point&,
+                          const gfx::Vector2d&,
+                          bool scrollByPage) = 0;
 
     // Stop scrolling the selected layer. Should only be called if scrollBegin()
     // returned ScrollStarted.