Bug 1292239 - Add utility to BrowserTestUtils to wait for load of selected browser in a brand new window. r=Felipe

MozReview-Commit-ID: IOChbNs8b3y

--HG--
extra : rebase_source : a778efe8b38aded85a6856a4c37c3b01ebd19894

testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm

@@ -204,6 +204,27 @@ this.BrowserTestUtils = {
     });
   },
 
+  /**
+   * Waits for the selected browser to load in a new window. This
+   * is most useful when you've got a window that might not have
+   * loaded its DOM yet, and where you can't easily use browserLoaded
+   * on gBrowser.selectedBrowser since gBrowser doesn't yet exist.
+   *
+   * @param {win}
+   *        A newly opened window for which we're waiting for the
+   *        first browser load.
+   *
+   * @return {Promise}
+   * @resolves Once the selected browser fires its load event.
+   */
+  firstBrowserLoaded(win) {
+    let mm = win.messageManager;
+    return this.waitForMessage(mm, "browser-test-utils:loadEvent", (msg) => {
+      let selectedBrowser = win.gBrowser.selectedBrowser;
+      return msg.target == selectedBrowser;
+    });
+  },
+
   /**
    * Waits for the web progress listener associated with this tab to fire a
    * STATE_STOP for the toplevel document.
@@ -433,11 +454,11 @@ this.BrowserTestUtils = {
     // Wait for browser-delayed-startup-finished notification, it indicates
     // that the window has loaded completely and is ready to be used for
     // testing.
-    yield this.waitForEvent(win, "load");
     let startupPromise =
       TestUtils.topicObserved("browser-delayed-startup-finished",
                               subject => subject == win).then(() => win);
-    let loadPromise = this.browserLoaded(win.gBrowser.selectedBrowser);
+
+    let loadPromise = this.firstBrowserLoaded(win);
 
     yield startupPromise;
     yield loadPromise;