Better workaround for extension browser test problems on win debug bots

chrome/test/data/extensions/api_test/webstore_private/theme.html

 <!--
  * Copyright (c) 2012 The Chromium Authors. All rights reserved.  Use of this
  * source code is governed by a BSD-style license that can be found in the
  * LICENSE file.
 -->
 <script src="common.js"></script>
 <script>
 
 var themeId = 'iamefpfkojoapidjnbafmgkgncegbkad';
 
 runTests([
   function installTheme() {
     listenOnce(chrome.management.onInstalled, function(info) {
       assertEq(info.id, themeId);
     });
 
-    var manifest = getManifest("../../../theme/manifest.json");
+    var manifest = getManifest("../../theme/manifest.json");
     chrome.webstorePrivate.beginInstallWithManifest3(
         {id: themeId, manifest: manifest},
         callbackPass(function(result) {
       assertEq("", result);
       chrome.webstorePrivate.completeInstall(themeId, callbackPass());
     }));
   }
 ]);
 
 </script>