[android_webview] Use a fraction to calculate scroll offset.

cc/input/layer_scroll_offset_delegate.h

Index: cc/input/layer_scroll_offset_delegate.h
diff --git a/cc/input/layer_scroll_offset_delegate.h b/cc/input/layer_scroll_offset_delegate.h
index 895d36d74908247cd3b2a984ed9d168a05a99180..dfe5be30ebe4e4c152f12b99ba73bc3556ea2a6c 100644
--- a/cc/input/layer_scroll_offset_delegate.h
+++ b/cc/input/layer_scroll_offset_delegate.h
@@ -16,6 +16,10 @@ namespace cc {
 // 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;
@@ -25,7 +29,8 @@ class LayerScrollOffsetDelegate {
   // 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: