Bundle main thread overscroll data with event ack messages

content/renderer/render_widget.h

Index: content/renderer/render_widget.h
diff --git a/content/renderer/render_widget.h b/content/renderer/render_widget.h
index 5d58a876efd8cf56759129979bd98a67c0d4e845..92a0f387f119804f52f04ef666589fb1036323d1 100644
--- a/content/renderer/render_widget.h
+++ b/content/renderer/render_widget.h
@@ -87,6 +87,7 @@ class RenderWidgetCompositor;
 class RenderWidgetTest;
 class ResizingModeSelector;
 struct ContextMenuParams;
+struct DidOverscrollParams;
 struct WebPluginGeometry;
 
 // RenderWidget provides a communication bridge between a WebWidget and
@@ -678,6 +679,12 @@ class CONTENT_EXPORT RenderWidget
   // Are we currently handling an input event?
   bool handling_input_event_;
 
+  // Used to intercept overscroll notifications while an event is being
+  // handled. If the event causes overscroll, the overscroll metadata can be
+  // bundled in the event ack, saving an IPC.  Note that we must continue
+  // supporting overscroll IPC notifications due to fling animation updates.
+  scoped_ptr<DidOverscrollParams>* handling_event_overscroll_;
+
   // Are we currently handling an ime event?
   bool handling_ime_event_;