Add overlay z-index variable

This allows for other themes to mutate the z-index. This is especially useful
in the embedded context.

Change-Id: I2a6e7938d118a662fc93c66155702d66749e59e6

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

@@ -356,6 +356,9 @@ limitations under the License.
           min-width: initial;
           width: 100vw;
         }
+        #replyOverlay {
+          z-index: var(--reply-overlay-z-index);
+        }
       }
     </style>
     <div class="container loading" hidden$="[[!_loading]]">Loading...</div>