Support choosing multiple files with fileSystem.chooseEntry.

chrome/test/data/extensions/api_test/file_system/open_multiple_writable_existing_with_write/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(
+        {acceptsMultiple: true, type: 'openWritableFile'},
+        chrome.test.callbackPass(function(entries) {
+          chrome.test.assertEq(2, entries.length);
+          checkEntry(entries[0], 'open_existing1.txt', false, true);
+          checkEntry(entries[1], 'open_existing2.txt', false, true);
+        })
+    );
+  }
+]);