Compile more Gallery targets in gyp v2.

ui/file_manager/gallery/js/image_editor/test_util.js

 // Copyright 2014 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.
 'use strict';
 
 /**
  * Creates a sample canvas.
  * @return {HTMLCanvasElement}
  */
 function getSampleCanvas() {
-  var canvas = document.createElement('canvas');
+  var canvas =
+      /** @type {HTMLCanvasElement} */ (document.createElement('canvas'));
   canvas.width = 1920;
   canvas.height = 1080;
 
   var ctx = canvas.getContext('2d');
   ctx.fillStyle = '#000000';
   for (var i = 0; i < 10; i++) {
     ctx.fillRect(i * 30, i * 30, 20, 20);
   }
 
   return canvas;
 }