Fix Bold font is not seen for Remove this Person text of Supervised User in CrOS

Not sure why it is missing. Probably related to this CL: https://codereview.chromium.org/2007133006

This fix is to add a style to .remove-warning-button.

BUG=631975

Review-Url: https://codereview.chromium.org/2197653003
Cr-Commit-Position: refs/heads/master@{#408750}

ui/login/account_picker/user_pod_row.css

@@ -673,6 +673,10 @@ html[dir=rtl] .action-box-area.active ~ .action-box-menu {
   text-align: end;
 }
 
+.remove-warning-button {
+  font-weight: bold;
+}
+
 /* Hide dialog elements not in a correct category. Only combinations currently
    in use are included here. */
 .pod.legacy-supervised .non-sync,