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>

polygerrit-ui/app/styles/themes/app-theme.html

@@ -133,6 +133,8 @@ limitations under the License.
   --syntax-selector-attr-color: #FA8602;
   --syntax-template-tag-color: #FA8602;
   --syntax-param-color: var(--primary-text-color);
+
+  --reply-overlay-z-index: 1000;
 }
 @media screen and (max-width: 50em) {
   :root {

polygerrit-ui/app/styles/themes/dark-theme.html

@@ -80,6 +80,8 @@
       --syntax-selector-attr-color: #80CBBF;
       --syntax-template-tag-color: #C792EA;
 
+      --reply-overlay-z-index: 1000;
+
       background-color: var(--view-background-color);
     }
   </style>