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

chrome/browser/resources/file_manager/foreground/js/file_grid.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';
 
 /**
  * FileGrid constructor.
  *
  * Represents grid for the Grid Vew in the File Manager.
@@ skipping 283 matching lines @@
     for (var horizontalIndex = horizontalStartIndex;
          horizontalIndex < horizontalEndIndex;
          horizontalIndex++) {
       var index = indexBase + horizontalIndex;
       if (0 <= index && index < this.dataModel.length)
         currentSelection.push(index);
     }
   }
   return currentSelection;
 };