Support choosing multiple files with fileSystem.chooseEntry.

chrome/test/data/extensions/api_test/file_system/open_multiple_writable_existing_with_write/test_util.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.
 
 // This is a duplicate of the file test_util in
 // chrome/test/data/extensions/api_test/file_system
 
 function checkEntry(entry, expectedName, isNew, shouldBeWritable) {
   chrome.test.assertEq(expectedName, entry.name);
   // Test that the file can be read.
@@ skipping 42 matching lines @@
         var blob = new Blob(["HoHoHo!"], {type: "text/plain"});
         fileWriter.write(blob);
       });
     });
     reader.onerror = chrome.test.callback(function(e) {
       chrome.test.fail("Error reading file contents.");
     });
     reader.readAsText(file);
   }));
 }