Support choosing multiple files with fileSystem.chooseEntry.

chrome/test/data/extensions/api_test/file_system/open_multiple_existing/test.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.
 
 chrome.test.runTests([
   function openFile() {
     chrome.fileSystem.chooseEntry(
-        {suggestedName: 'open_existing.txt'},
-        chrome.test.callbackPass(function(entry) {
-          checkEntry(entry, 'open_existing.txt', false, false);
+        {acceptsMultiple: true},
+        chrome.test.callbackPass(function(entries) {
+          checkEntry(entries[0], 'open_existing1.txt', false, false);
+          checkEntry(entries[1], 'open_existing2.txt', false, false);
         })
     );
   }
 ]);