[filebrowser] Show progress indication, while files from GData are being downloaded.

chrome/browser/resources/file_manager/js/mock_chrome.js

 // Copyright (c) 2012 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.
 
 function MockEventSource() {
   this.listeners_ = [];
 }
 
 MockEventSource.prototype.addListener = function(listener) {
   this.listeners_.push(listener);
@@ skipping 347 matching lines @@
       PREVIEW_COLUMN_LABEL: 'Preview',
       OFFILE_COLUMN_LABEL: 'Available offline',
 
       ERROR_CREATING_FOLDER: 'Unable to create folder "$1": $2',
       ERROR_INVALID_CHARACTER: 'Invalid character: $1',
       ERROR_RESERVED_NAME: 'This name may not be used as a file of folder name',
       ERROR_WHITESPACE_NAME: 'Invalid name',
       ERROR_NEW_FOLDER_EMPTY_NAME: 'Please specify a folder name',
       NEW_FOLDER_BUTTON_LABEL: 'New folder',
       FILENAME_LABEL: 'File Name',
+      PREPARING_LABEL: 'Preparing',
 
       DIMENSIONS_LABEL: 'Dimensions',
       DIMENSIONS_FORMAT: '$1 x $2',
 
       EJECT_BUTTON: 'Eject',
       IMAGE_DIMENSIONS: 'Image Dimensions',
       VOLUME_LABEL: 'Volume Label',
       READ_ONLY: 'Read Only',
 
       PLAY_MEDIA: 'Play',
@@ skipping 205 matching lines @@
 
   setWindowHeight: function(height) {
     this.popup_.style.height = height + 'px';
   },
 
   closeWindow: function() {
     this.popup_.parentNode.removeChild(this.popup_);
     this.popup_ = null;
   }
 };