Add plugin stub for print preview tests

chrome/test/data/webui/print_preview/plugin_stub.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.
+
+cr.define('print_preview', function() {
+  /**
+  * Test version of the print preview plugin
+  * @param {print_preview.PreviewArea} The PreviewArea that owns this plugin.

[dpapad, 2017/07/06 00:24:17]

!print_preview.PreviewArea

[rbpotter, 2017/07/06 01:57:27]

Done.

+  * @constructor
+  */
+  function PDFPluginStub(area) {

[dpapad, 2017/07/06 00:24:17]

Can we use the new ES6 class syntax, which we recently allowed in the styleguide
(see [1])? See example CL at https://codereview.chromium.org/2960293002.

While we are not aggressively converting all existing code to use it, it makes
sense to use it in new code.

[1]
https://chromium.googlesource.com/chromium/src/+/286a6b8e56a7bd0b7ac58d8b9ba9...

[rbpotter, 2017/07/06 01:57:28]

Done.

+    /** @private {Object} The callback to run when the plugin has loaded. */

[dpapad, 2017/07/06 00:24:17]

?Function

[rbpotter, 2017/07/06 01:57:27]

Done.

+    this.loadCallback_ = null;
+
+    /** @private {EventTracker} The plugin stub's event tracker. */

[dpapad, 2017/07/06 00:24:17]

!EventTracker

[rbpotter, 2017/07/06 01:57:28]

Done.

+    this.tracker_ = new EventTracker();
+
+    // Call documentLoadComplete as soon as the preview area starts the preview.
+    this.tracker_.add(
+        area,
+        print_preview.PreviewArea.EventType.PREVIEW_GENERATION_IN_PROGRESS,
+        this.documentLoadComplete.bind(this));
+  }
+
+  PDFPluginStub.prototype = {
+    /**
+     * @param {Object} callback The callback to run when the plugin has loaded.
+     */
+    setLoadCallback: function(callback) {
+      this.loadCallback_ = callback;
+    },
+
+    documentLoadComplete: function() {
+      if (this.loadCallback_)
+        this.loadCallback_();
+    },
+
+    /**
+     * Stubbed out since some tests result in a call.
+     * @param {string} url The url to initialize the plugin to.
+     * @param {boolean} color Whether the preview should be in color.
+     * @param {Array<int>} pages The pages to preview.

[dpapad, 2017/07/06 00:24:17]

!Array<number>

[rbpotter, 2017/07/06 01:57:28]

Done.

+     * @param {boolean} modifiable Whether the source document is modifiable.
+     */
+    resetPrintPreviewMode: function(url, color, pages, modifiable) {},
+  };
+
+  return {
+    PDFPluginStub: PDFPluginStub,
+  };
+});