Make shelf item can be dragged when context menu is opened.

ui/views/controls/menu/menu_host.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_VIEWS_CONTROLS_MENU_MENU_HOST_H_
 #define UI_VIEWS_CONTROLS_MENU_MENU_HOST_H_
 
 #include <memory>
 
 #include "base/compiler_specific.h"
@@ skipping 60 matching lines @@
   friend class test::MenuControllerTest;
 
   // Overridden from Widget:
   internal::RootView* CreateRootView() override;
   void OnMouseCaptureLost() override;
   void OnNativeWidgetDestroyed() override;
   void OnOwnerClosing() override;
   void OnDragWillStart() override;
   void OnDragComplete() override;
 
+  // Parent of the MenuHost widget.
+  Widget* owner_ = nullptr;
+
   // The view we contain.
   SubmenuView* submenu_;
 
   // If true, DestroyMenuHost has been invoked.
   bool destroying_;
 
   // If true and capture is lost we don't notify the delegate.
   bool ignore_capture_lost_;
 
 #if !defined(OS_MACOSX)
   // Handles raw touch events at the moment.
   std::unique_ptr<internal::PreMenuEventDispatchHandler> pre_dispatch_handler_;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(MenuHost);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_CONTROLS_MENU_MENU_HOST_H_