Add window.IN_TEST in gallery/js/background.js.

ui/file_manager/gallery/js/background.js

 // Copyright 2014 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.
 
 /**
  * @param {!Object.<string, string>} stringData String data.
  * @param {!VolumeManager} volumeManager Volume manager.
  * @constructor
  * @struct
  */
@@ skipping 204 matching lines @@
 
     launch(selectedEntriesPromise).catch(function(error) {
       console.error(error.stack || error);
     });
   });
 }
 
 // If is is run in the browser test, wait for the test resources are installed
 // as a component extension, and then load the test resources.
 if (chrome.test) {
+  // Sets a global flag that we are in tests, so other components are aware of
+  // it.
+  window.IN_TEST = true;
+
   /** @type {string} */
   window.testExtensionId = 'ejhcmmdhhpdhhgmifplfmjobgegbibkn';
   chrome.runtime.onMessageExternal.addListener(function(message) {
     if (message.name !== 'testResourceLoaded')
       return;
     var script = document.createElement('script');
     script.src =
         'chrome-extension://' + window.testExtensionId +
         '/common/test_loader.js';
     document.documentElement.appendChild(script);
   });
 }