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
0e3631dcdd