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

[dpapad, 2017/07/06 16:42:24]

Should we ever be calling setLoadCallback(null)? If not let's update this to be
!Function

[rbpotter, 2017/07/06 19:47:29]

Done. I was thinking maybe we would want to clear it sometimes, but
currently there is no use for this. Can always change it back later
if this needs to happen for some reason.

+     *     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};
+});