[Files.app] Split the JavaScript files into subdirectories: common, background, and foreground

chrome/browser/resources/file_manager/foreground/js/photo/gallery.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.
 
 'use strict';
 
 /**
  * Called from the main frame when unloading.
  * @return {string?} User-visible message on null if it is OK to close.
  */
@@ skipping 850 matching lines @@
  */
 Gallery.prototype.updateButtons_ = function() {
   if (this.modeButton_) {
     var oppositeMode =
         this.currentMode_ == this.slideMode_ ? this.mosaicMode_ :
                                                this.slideMode_;
     this.modeButton_.title =
         this.displayStringFunction_(oppositeMode.getTitle());
   }
 };