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

ui/views/controls/menu/menu_host.h

Index: ui/views/controls/menu/menu_host.h
diff --git a/ui/views/controls/menu/menu_host.h b/ui/views/controls/menu/menu_host.h
index 484b90889320a3c7c05724aacfbab234382db741..b49f58930e566355b0c3f954b5b0d94c11e00506 100644
--- a/ui/views/controls/menu/menu_host.h
+++ b/ui/views/controls/menu/menu_host.h
@@ -11,6 +11,7 @@
 #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 {
 
@@ -36,7 +37,7 @@ class MenuControllerTest;
 // 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;
@@ -70,7 +71,7 @@ class MenuHost : public Widget {
  private:
   friend class test::MenuControllerTest;
 
-  // Overridden from Widget:
+  // Widget:
   internal::RootView* CreateRootView() override;
   void OnMouseCaptureLost() override;
   void OnNativeWidgetDestroyed() override;
@@ -78,6 +79,12 @@ class MenuHost : public Widget {
   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_;