[Files.app] Fix a potential error in ShareDialog/FileManagerBrowserTest

chrome/test/data/extensions/api_test/file_manager_browsertest/test_cases.js

 // Copyright (c) 2013 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.
 
 'use strict';
 
 /**
  * @enum {string}
  * @const
  */
@@ skipping 964 matching lines @@
     },
     // Wait until the share dialog's contents are shown.
     function(result) {
       chrome.test.assertTrue(result);
       callRemoteTestUtil('waitForElement',
                          appId,
                          ['.share-dialog-webview-wrapper.loaded'],
                          this.next);
     },
     function(result) {
+      chrome.test.assertTrue(!!result);
       callRemoteTestUtil('waitForStyles',
                          appId,
                          [{
                             query: '.share-dialog-webview-wrapper.loaded',
                             styles: {
                               width: '350px',
                               height: '250px'
                             }
                           }],
                          this.next);
     },
     // Wait until the share dialog's contents are shown.
     function(result) {
       callRemoteTestUtil('executeScriptInWebView',
                          appId,
                          ['.share-dialog-webview-wrapper.loaded webview',
                           'document.querySelector("button").click()'],
                          this.next);
     },
     // Wait until the share dialog's contents are hidden.
     function(result) {
+      chrome.test.assertTrue(!!result);
       callRemoteTestUtil('waitForElement',
                          appId,
                          ['.share-dialog-webview-wrapper.loaded',
                           null,   // iframeQuery
                           true],  // inverse
                          this.next);
     },
     // Check for Javascript errros.
     function() {
       checkIfNoErrorsOccured(this.next);
@@ skipping 761 matching lines @@
                          this.next);
     },
     // Verify the thumbnail.
     function(element) {
       chrome.test.assertTrue(element.attributes.src.indexOf(
           'data:image/jpeg') === 0);
       checkIfNoErrorsOccured(this.next);
     }
   ]);
 };