Bug 1479125 - Update marionette driver to access tabBrowser.tabs as an iterable;r=ato

MozReview-Commit-ID: 5CLK6V3u0iG

--HG--
extra : rebase_source : 2a21683e6b5f5dce7841e01d2687963820a3d32a

testing/marionette/driver.js

@@ -262,12 +262,12 @@ Object.defineProperty(GeckoDriver.prototype, "windowHandles", {
 
       // Only return handles for browser windows
       if (tabBrowser && tabBrowser.tabs) {
-        tabBrowser.tabs.forEach(tab => {
+        for (let tab of tabBrowser.tabs) {
           let winId = this.getIdForBrowser(browser.getBrowserForTab(tab));
           if (winId !== null) {
             hs.push(winId);
           }
-        });
+        }
       }
     }