Bug 1315887 - Re-enable browser_locationBarExternalLoad.js by making it less race-y. r=mak

Differential Revision: https://phabricator.services.mozilla.com/D14192

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

browser/base/content/test/urlbar/browser.ini

@@ -44,7 +44,6 @@ support-files =
 [browser_dragdropURL.js]
 [browser_locationBarCommand.js]
 [browser_locationBarExternalLoad.js]
-skip-if = true # Bug 1315887
 [browser_moz_action_link.js]
 [browser_new_tab_urlbar_reset.js]
 [browser_page_action_menu.js]

browser/base/content/test/urlbar/browser_locationBarExternalLoad.js

@@ -45,9 +45,12 @@ async function testURL(url, loadFunc, endFunc) {
   let browser = tab.linkedBrowser;
 
   let pagePrincipal = gBrowser.contentPrincipal;
+  // We need to ensure that we set the pageshow event listener before running
+  // loadFunc, otherwise there's a chance that the content process will finish
+  // loading the page and fire pageshow before the event listener gets set.
+  let pageShowPromise = BrowserTestUtils.waitForContentEvent(browser, "pageshow");
   loadFunc(url);
-
-  await BrowserTestUtils.waitForContentEvent(browser, "pageshow");
+  await pageShowPromise;
 
   await ContentTask.spawn(browser, { isRemote: gMultiProcessBrowser },
     async function(arg) {