[Notifications] Fix swipe to close for ARC notifications.

ui/message_center/views/slide_out_controller.h

Index: ui/message_center/views/slide_out_controller.h
diff --git a/ui/message_center/views/slide_out_controller.h b/ui/message_center/views/slide_out_controller.h
index 6d7a71c7e3bc0c4fe14cfcf6e5938c75d4202808..5b01df2d98ea8fd9b7f218be216a6bdbf41ac827 100644
--- a/ui/message_center/views/slide_out_controller.h
+++ b/ui/message_center/views/slide_out_controller.h
@@ -23,6 +23,9 @@ class SlideOutController : public ui::EventHandler,
     // Returns the layer for slide operations.
     virtual ui::Layer* GetSlideOutLayer() = 0;
 
+    // Called when a slide starts, ends, or is updated.
+    virtual void OnSlideChanged() = 0;
+
     // Called when user intends to close the View by sliding it out.
     virtual void OnSlideOut() = 0;
   };