Bug 1820509 - Tweak compat intervention to use -webkit-box-flex rather than -moz-box-flex. r=webcompat-reviewers,denschub

Use -webkit-box-flex rather than -moz-box-flex, so this doesn't stop
working when we stop supporting the -moz prefixed-properties.

I noticed this while switching our front-end away from -moz-box-flex.

Differential Revision: https://phabricator.services.mozilla.com/D171700

browser/extensions/webcompat/injections/css/bug1694470-myvidster.com-content-not-shown.css

@@ -4,8 +4,8 @@
  * WebCompat issue #67308 - https://webcompat.com/issues/67308
  *
  * The site depends on Sencha Touch and should receive some specific
- * -moz-box-flex to be working.
+ * -webkit-box-flex be working.
  */
 #home_refresh_var {
-  -moz-box-flex: 1;
+  -webkit-box-flex: 1;
 }