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 PDF plugin
+   */
+  class PDFPluginStub {
+    /**
+     * @param {!print_preview.PreviewArea} The PreviewArea that owns this
+     *     plugin.
+     */
+    constructor(area) {
+      /**
+       * @private {?Function} The callback to run when the plugin has loaded.
+       */
+      this.loadCallback_ = null;
+
+      /** @private {!EventTracker} The plugin stub's event tracker. */
+      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));
+    }
+
+    /**
+     * @param {!Function} callback The callback to run when the plugin has
+     *     loaded.
+     */
+    setLoadCallback(callback) {
+      this.loadCallback_ = callback;
+    }
+
+    documentLoadComplete() {
+      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<number>} pages The pages to preview.
+     * @param {boolean} modifiable Whether the source document is modifiable.
+     */
+    resetPrintPreviewMode(url, color, pages, modifiable) {}
+  }
+
+  return {PDFPluginStub: PDFPluginStub};
+});