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 0fc01bf50fa818cb6fc8c0dec21fc605cc27771c..588f0ee25408675fe48f4c1656ed204c5038f9cc 100644
--- a/ash/shelf/shelf_button.cc
+++ b/ash/shelf/shelf_button.cc
@@ -457,6 +457,10 @@ void ShelfButton::OnGestureEvent(ui::GestureEvent* event) {
       shelf_view_->PointerReleasedOnButton(this, ShelfView::TOUCH, false);
       event->SetHandled();
       return;
+    case ui::ET_GESTURE_LONG_TAP:
+      // Handle LONG_TAP to avoid opening the context menu twice.
+      event->SetHandled();
+      return;
     default:
       return CustomButton::OnGestureEvent(event);
   }