Add plugin stub for print preview tests

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

 // Copyright 2016 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.
 
 cr.define('print_preview_test', function() {
   /**
    * Index of the "Save as PDF" printer.
    * @type {number}
    * @const
    */
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       localDestinationInfos = [
         { printerName: 'FooName', deviceName: 'FooDevice' },
         { printerName: 'BarName', deviceName: 'BarDevice' },
       ];
 
       nativeLayer = new print_preview.NativeLayerStub();
       print_preview.NativeLayer.setInstance(nativeLayer);
       printPreview = new print_preview.PrintPreview();
       previewArea = printPreview.getPreviewArea();
-      previewArea.setIsBrowserTest(true);
+      previewArea.plugin_ =
+          new print_preview.PDFPluginStub(previewArea);
+      previewArea.plugin_.setLoadCallback(
+          previewArea.onPluginLoad_.bind(previewArea));
     });
 
     // Test some basic assumptions about the print preview WebUI.
     test('PrinterList', function() {
       return setupSettingsAndDestinationsWithCapabilities().then(function() {
         var recentList =
             $('destination-search').querySelector('.recent-list ul');
         var localList =
             $('destination-search').querySelector('.local-list ul');
         assertNotEquals(null, recentList);
@@ skipping 1099 matching lines @@
       return setupSettingsAndDestinationsWithCapabilities().then(function() {
         // The system default destination should be used instead of the
         // most recent destination.
         assertEquals(
             'FooDevice',
             printPreview.destinationStore_.selectedDestination.id);
       });
     });
   });
 });