Bug 1260312 - Retry mochitest-chrome redirection to improve reliability; r=jmaher

The mochitest-chrome harness sends a custom "contentEvent" event from redirect.html
to a listener in browser-test.js. It is possible for redirect.html to be loaded
and send contentEvent before the listener is set up in browser-test.js. In the
absence of a better synchronization strategy, redirect.html now retries the send
after a few seconds. If the first contentEvent was received, the second will be
ignored; if the first contentEvent was not received, the second should avoid an
intermittent harness hang and timeout.

testing/mochitest/redirect.html

@@ -27,6 +27,8 @@
       // added until then.
       window.addEventListener("MozAfterPaint", function() {
         setTimeout(redirectToHarness, 0);
+        // In case the listener was not ready, try again after a few seconds.
+        setTimeout(redirectToHarness, 5000);
       }, {once: true});
 
     }