Add placeholder element to fixtures/ssr to keep it focused in DevTools (

facebook#10085)

When going through the fixtures one by one it's convenient to see the server-side rendered markup in the Chrome DevTools Elements along with the generated warning in Console. The placeholder element adds consistency to the markup: it does not disappear with page reloads, so Chrome DevTools is smart enough to keep it focused.

fixtures/ssr/src/components/SSRMismatchTest.js

@@ -324,7 +324,9 @@ export default class SSRMismatchTest extends Component {
             ))}
           </ol>
         </div>
-        {content}
+        <div className="SSRMismatchTest__placeholder">
+          {content}
+        </div>
       </div>
     );
   }