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

cc/input_handler.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_INPUT_HANDLER_H_
 #define CC_INPUT_HANDLER_H_
 
 #include "base/basictypes.h"
 #include "base/time.h"
 #include "cc/cc_export.h"
@@ skipping 28 matching lines @@
 
     // Scroll the selected layer starting at the given position. If the scroll
     // type given to scrollBegin was a gesture, then the scroll point and delta
     // should be in viewport (logical pixel) coordinates. Otherwise they are in
     // scrolling layer's (logical pixel) space. If there is no room to move the
     // layer in the requested direction, its first ancestor layer that can be
     // scrolled will be moved instead. If no layer can be moved in the requested
     // 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.
     virtual void scrollEnd() = 0;
 
     virtual void pinchGestureBegin() = 0;
     virtual void pinchGestureUpdate(float magnifyDelta, gfx::Point anchor) = 0;
     virtual void pinchGestureEnd() = 0;
 
     virtual void startPageScaleAnimation(gfx::Vector2d targetOffset,
@@ skipping 26 matching lines @@
 protected:
     InputHandler() { }
 
 private:
     DISALLOW_COPY_AND_ASSIGN(InputHandler);
 };
 
 }
 
 #endif  // CC_INPUT_HANDLER_H_