Add color: inherit to gr-date-formatter

As a consequence of 45e9c0d, gr-date-formatter has its styles
overridden by other rules.

This change adds color: inherit to ensure that if a parent styles it
(sometimes implicitly, like by wrapping it in an <a> tag), the style is
properly applied.

Change-Id: I32a215502da8c2600b52e43b6f856c7e01d3e1bf

polygerrit-ui/app/elements/shared/gr-date-formatter/gr-date-formatter.html

@@ -27,6 +27,7 @@ limitations under the License.
   <template>
     <style include="shared-styles">
       :host {
+        color: inherit;
         display: inline;
       }
     </style>