Allow menus to stay open during a child view's drag and drop

ui/views/widget/widget.h

Index: ui/views/widget/widget.h
diff --git a/ui/views/widget/widget.h b/ui/views/widget/widget.h
index bf9ce093650d16f37f86af7e73fac2c453dfd4ef..6f85324bdd9325ba00f38ad5cb8ed777fee30fc1 100644
--- a/ui/views/widget/widget.h
+++ b/ui/views/widget/widget.h
@@ -793,6 +793,12 @@ class VIEWS_EXPORT Widget : public internal::NativeWidgetDelegate,
   // TODO(beng): remove once we fold those objects onto this one.
   void DestroyRootView();
 
+  // Notification that a drag will start. Default implementation does nothing.
+  virtual void OnDragWillStart();
+
+  // Notification that the drag performed by RunShellDrag() has completed.
+  virtual void OnDragComplete();
+
  private:
   friend class ComboboxTest;
   friend class TextfieldTest;