Bug 1507689 - Fix an intermittent failure condition in test_WebExtensionContentScript.js which is turned into a perma-failure with the patch in bug 1504728 r=kmag

Differential Revision: https://phabricator.services.mozilla.com/D12093

--HG--
extra : moz-landing-system : lando

toolkit/components/extensions/test/xpcshell/test_WebExtensionContentScript.js

@@ -57,10 +57,12 @@ async function loadURL(url, {frameCount}) {
     }
   }
   function loadObserver(window) {
+    window.addEventListener("load", function onLoad() {
       windows.set(window.location.href, window);
       if (windows.size == frameCount) {
         resolveLoad();
       }
+    }, {once: true});
   }
 
   Services.obs.addObserver(requestObserver, "http-on-examine-response");