Fix reply dialog dropdown obstruction

Previously, we were applying --layout-scroll, which is defined in
iron-flex-layout to the reply dialog:

https://github.com/PolymerElements/iron-flex-layout/blob/master/iron-flex-layout.html#L364

-webkit-overflow-scrolling: touch definition seemed to cause the z-index
of gr-autocomplete to not be respected. Instead, just apply
overflow: auto;

Bug: Issue 7066
Change-Id: Ic567e734f1e3be956b9673e1691a6e768f8f1859

polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.html

@@ -324,7 +324,7 @@ limitations under the License.
           margin-right: 0;
         }
         .scrollable {
-          @apply(--layout-scroll);
+          overflow: auto;
         }
       }
     </style>