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"
 #include "base/macros.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/views/widget/widget.h"
+#include "ui/views/widget/widget_observer.h"
 
 namespace views {
 
 class SubmenuView;
 class View;
 class Widget;
 
 namespace internal {
 
 // This class is internal to views.
 class PreMenuEventDispatchHandler;
 
 }  // internal
 
 namespace test {
 class MenuControllerTest;
 }  // test
 
 // SubmenuView uses a MenuHost to house the SubmenuView.
 //
 // SubmenuView owns the MenuHost. When SubmenuView is done with the MenuHost
 // |DestroyMenuHost| is invoked. The one exception to this is if the native
 // OS destroys the widget out from under us, in which case |MenuHostDestroyed|
 // is invoked back on the SubmenuView and the SubmenuView then drops references
 // to the MenuHost.
-class MenuHost : public Widget {
+class MenuHost : public Widget, public WidgetObserver {
  public:
   explicit MenuHost(SubmenuView* submenu);
   ~MenuHost() override;
 
   // Initializes and shows the MenuHost.
   // WARNING: |parent| may be NULL.
   void InitMenuHost(Widget* parent,
                     const gfx::Rect& bounds,
                     View* contents_view,
                     bool do_capture);
@@ skipping 13 matching lines @@
 
   // Sets the bounds of the menu host.
   void SetMenuHostBounds(const gfx::Rect& bounds);
 
   // Releases a mouse grab installed by |ShowMenuHost|.
   void ReleaseMenuHostCapture();
 
  private:
   friend class test::MenuControllerTest;
 
-  // Overridden from Widget:
+  // Widget:
   internal::RootView* CreateRootView() override;
   void OnMouseCaptureLost() override;
   void OnNativeWidgetDestroyed() override;
   void OnOwnerClosing() override;
   void OnDragWillStart() override;
   void OnDragComplete() override;
 
+  // WidgetObserver:
+  void OnWidgetDestroying(Widget* widget) 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_