Add plugin stub for print preview tests

chrome/test/data/webui/print_preview/print_preview_ui_browsertest.js

 // Copyright 2017 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.
 
 /** @fileoverview Runs the Print Preview tests. */
 
 var ROOT_PATH = '../../../../../';
 
 /**
  * @constructor
@@ skipping 40 matching lines @@
 
   extraLibraries: [
     ROOT_PATH + 'ui/webui/resources/js/cr.js',
     ROOT_PATH + 'ui/webui/resources/js/promise_resolver.js',
     ROOT_PATH + 'third_party/mocha/mocha.js',
     ROOT_PATH + 'chrome/test/data/webui/mocha_adapter.js',
     ROOT_PATH + 'ui/webui/resources/js/util.js',
     ROOT_PATH + 'chrome/test/data/webui/test_browser_proxy.js',
     'print_preview_tests.js',
     'native_layer_stub.js',
+    'plugin_stub.js',
   ],
 };
 
 // Run each mocha test in isolation (within a new TEST_F() call).
 [
   'PrinterList',
   'PrinterListCloudEmpty',
   'RestoreLocalDestination',
   'RestoreMultipleDestinations',
   'DefaultDestinationSelectionRules',
@@ skipping 43 matching lines @@
     mocha.grep(new RegExp(testName + '\\b')).run();
   });
 });
 
 GEN('#if !defined(OS_CHROMEOS)');
 TEST_F('PrintPreviewUIBrowserTest', 'SystemDefaultPrinterPolicy', function() {
   loadTimeData.overrideValues({useSystemDefaultPrinter: true});
   mocha.grep(new RegExp('SystemDefaultPrinterPolicy' + '\\b')).run();
 });
 GEN('#endif');