Bug 1414136 - Fix test failures in session history relying on non-comformant Promise handling. r=sawang

docshell/test/navigation/file_bug1326251.html

@@ -154,7 +154,7 @@
         frame.addEventListener('load', function listener(e) {
           if (++count == occurances) {
             frame.removeEventListener('load', listener);
-            resolve();
+            setTimeout(resolve, 0);
           }
         });
       });

docshell/test/navigation/file_bug1364364-1.html

@@ -10,7 +10,7 @@
     </div>
     <script type="application/javascript">
     function waitForLoad(frame) {
-      return new Promise(r => frame.onload = r);
+      return new Promise(r => frame.onload = () => setTimeout(r, 0));
     }
 
     async function loadFramesAndNavigate() {