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..fdecd41288458024cab624f384712417dca377f7 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);
   }