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 53cf800bbb5e92de9455233308ccf9c3be89871c..dd81805c7b98bb3490d830ad086db3fb7d6e4fd9 100644
--- a/ash/shelf/shelf_button.cc
+++ b/ash/shelf/shelf_button.cc
@@ -461,6 +461,9 @@ void ShelfButton::OnGestureEvent(ui::GestureEvent* event) {
       shelf_view_->PointerReleasedOnButton(this, ShelfView::TOUCH, false);
       event->SetHandled();
       return;
+    case ui::ET_GESTURE_LONG_TAP:
+      event->SetHandled();
+      return;
     default:
       return CustomButton::OnGestureEvent(event);
   }