Update platform_app.js to work even if window.history is not replaceable

chrome/test/data/extensions/platform_apps/restrictions/main.js

 // 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.
 
 var assertEq = chrome.test.assertEq;
 var assertTrue = chrome.test.assertTrue;
 var fail = chrome.test.fail;
 var succeed = chrome.test.succeed;
 
 function assertThrowsError(method, opt_expectedError) {
@@ skipping 47 matching lines @@
     assertEq('undefined', typeof(document.linkColor));
     assertEq('undefined', typeof(document.vlinkColor));
 
     succeed();
   },
 
   function testHistory() {
     // Accessing these logs warnings to the console.
     assertEq('undefined', typeof(history.back));
     assertEq('undefined', typeof(history.forward));
+    assertEq('undefined', typeof(history.go));
     assertEq('undefined', typeof(history.length));
 
     // These are part of the HTML5 History API that are feature detected, so we
     // remove them altogether, allowing apps to have fallback behavior.
     chrome.test.assertFalse('pushState' in history);
     chrome.test.assertFalse('replaceState' in history);
     chrome.test.assertFalse('state' in history);
 
     succeed();
   },
@@ skipping 109 matching lines @@
     assertEq('undefined', typeof(chrome.extension));
     succeed();
   },
 
   function testExtensionApis() {
     assertEq('undefined', typeof(chrome.tabs));
     assertEq('undefined', typeof(chrome.windows));
     succeed();
   }
 ]);