Bug 1284102 - Fix a race in test code. r=florian

MozReview-Commit-ID: 95xu305OBBM

--HG--
extra : rebase_source : f80ef514ba8c57fb2c4535e83ab15285249d5c2f
extra : source : 2ce2e4c2909ffaf94616b19c3cbfe7d29ad10036

browser/base/content/test/webrtc/browser_devices_get_user_media_anim.js

@@ -58,6 +58,10 @@ var gTests = [
 
       // And finally verify the attribute is removed when closing the stream.
       yield closeStream();
+
+      // TODO(Bug 1304997): Fix the race in closeStream() and remove this
+      // promiseWaitForCondition().
+      yield promiseWaitForCondition(() => !tab.getAttribute("sharing"));
       is(tab.getAttribute("sharing"), "",
          "the tab no longer has the 'sharing' attribute after closing the stream");
     }