[android_webview] Use a fraction to calculate scroll offset.

cc/input/layer_scroll_offset_delegate.h

 // Copyright 2013 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_LAYER_SCROLL_OFFSET_DELEGATE_H_
 #define CC_INPUT_LAYER_SCROLL_OFFSET_DELEGATE_H_
 
 #include "base/basictypes.h"
 #include "ui/gfx/vector2d_f.h"
 
 namespace cc {
 
 // The LayerScrollOffsetDelegate allows for the embedder to take ownership of
 // the scroll offset of the root layer.
 //
 // The LayerScrollOffsetDelegate is only used on the impl thread.
 class LayerScrollOffsetDelegate {
  public:
+  // This is called by the compositor to notify the delegate what is the upper
+  // total scroll offset bound.
+  virtual void SetMaxScrollOffset(gfx::Vector2dF max_scroll_offset) = 0;
+
   // This is called by the compositor when the scroll offset of the layer would
   // have otherwise changed.
   virtual void SetTotalScrollOffset(gfx::Vector2dF new_value) = 0;
 
   // This is called by the compositor to query the current scroll offset of the
   // layer.
   // There is no requirement that the return values of this method are
   // stable in time (two subsequent calls may yield different results).
   // The return value is not required to be related to the values passed in to
-  // the SetTotalScrollOffset method in any way.
+  // the SetTotalScrollOffset method in any way however it is required to be no
+  // more than the value passed to the most recent SetMaxScrollOffset call.
   virtual gfx::Vector2dF GetTotalScrollOffset() = 0;
 
  protected:
   LayerScrollOffsetDelegate() {}
   virtual ~LayerScrollOffsetDelegate() {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(LayerScrollOffsetDelegate);
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_LAYER_SCROLL_OFFSET_DELEGATE_H_