Add isWritableFileEntry to the fileSystem API

chrome/renderer/resources/extensions/file_system_custom_bindings.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.
 
 // Custom bindings for the fileSystem API.
 
 var chromeHidden = requireNative('chrome_hidden').GetChromeHidden();
 var fileSystemNatives = requireNative('file_system_natives');
 var GetIsolatedFileSystem = fileSystemNatives.GetIsolatedFileSystem;
 var lastError = require('lastError');
 
 chromeHidden.registerCustomHook('fileSystem', function(bindingsAPI) {
   var apiFunctions = bindingsAPI.apiFunctions;
   function bindFileEntryFunction(functionName) {
     apiFunctions.setUpdateArgumentsPostValidate(
         functionName, function(fileEntry, callback) {
       var fileSystemName = fileEntry.filesystem.name;
       var relativePath = fileEntry.fullPath.slice(1);
       return [fileSystemName, relativePath, callback];
     });
   }
-  ['getDisplayPath', 'getWritableFileEntry'].forEach(bindFileEntryFunction);
+  ['getDisplayPath', 'getWritableFileEntry', 'isWritableFileEntry']
+      .forEach(bindFileEntryFunction);
 
   function bindFileEntryCallback(functionName) {
     apiFunctions.setCustomCallback(functionName,
         function(name, request, response) {
       if (request.callback && response) {
         var callback = request.callback;
         request.callback = null;
 
         var fileSystemId = response.fileSystemId;
         var baseName = response.baseName;
         var fs = GetIsolatedFileSystem(fileSystemId);
 
         try {
           fs.root.getFile(baseName, {}, function(fileEntry) {
             callback(fileEntry);
           }, function(fileError) {
             lastError.set('Error getting fileEntry, code: ' + fileError.code);
             callback();
           });
         } catch (e) {
           lastError.set('Error in event handler for onLaunched: ' + e.stack);
           callback();
         }
       }
     });
   }
   ['getWritableFileEntry', 'chooseFile'].forEach(bindFileEntryCallback);
 });