Improve the avatar load in flicker

Since there is a delay between the background image loading and the
initial element being displayed, providing a background color makes it
seem like something is "supposed" to be there, thus making the load in
experience less jarring.

Change-Id: Ie0529dfb439ca02e45f94fbf197cdd388d6d41f4

polygerrit-ui/app/elements/gr-avatar.html

@@ -23,6 +23,7 @@ limitations under the License.
         display: inline-block;
         border-radius: 50%;
         background-size: cover;
+        background-color: var(--background-color, #f1f2f3);
       }
     </style>
   </template>