Plumb an overscroll callback form the compositor to the Android UI thread.

content/browser/android/content_view_core_impl.h

Index: content/browser/android/content_view_core_impl.h
diff --git a/content/browser/android/content_view_core_impl.h b/content/browser/android/content_view_core_impl.h
index d59bece1612eb4b5e28715be61f61d5b1b9fd21d..3bcc3313d7275eaf2282a0c89a8714a18b302d97 100644
--- a/content/browser/android/content_view_core_impl.h
+++ b/content/browser/android/content_view_core_impl.h
@@ -229,6 +229,7 @@ class ContentViewCoreImpl : public ContentViewCore,
   void OnTabCrashed();
   void UpdateContentSize(int width, int height);
   void UpdateScrollOffsetAndPageScaleFactor(int x, int y, float scale);
+  void UpdateUnusedScrollOffset(int x, int y);
   void UpdatePageScaleLimits(float minimum_scale, float maximum_scale);
   void UpdateOffsetsForFullscreen(float controls_offset_y,
                                   float content_offset_y);