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

ash/shelf/shelf_button.cc

Index: ash/shelf/shelf_button.cc
diff --git a/ash/shelf/shelf_button.cc b/ash/shelf/shelf_button.cc
index f8c930d29ce9c4c65a5441bc013f0b3692b0ba3f..ae8dd4ddcd29c54931909fa79d365bb89a32325b 100644
--- a/ash/shelf/shelf_button.cc
+++ b/ash/shelf/shelf_button.cc
@@ -26,6 +26,7 @@
 #include "ui/views/animation/ink_drop_impl.h"
 #include "ui/views/animation/square_ink_drop_ripple.h"
 #include "ui/views/controls/image_view.h"
+#include "ui/views/controls/menu/menu_controller.h"
 
 namespace {
 
@@ -461,6 +462,10 @@ void ShelfButton::OnGestureEvent(ui::GestureEvent* event) {
       shelf_view_->PointerReleasedOnButton(this, ShelfView::TOUCH, false);
       event->SetHandled();
       return;
+    case ui::ET_GESTURE_LONG_TAP:
+      views::MenuController::SetDoCancel(true);
+      event->SetHandled();
+      return;
     default:
       return CustomButton::OnGestureEvent(event);
   }