[fsp] Enable the File System Provider API for extensions.

chrome/test/data/extensions/api_test/file_system_provider/extension/test.js

+// Copyright 2014 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';
+
+chrome.test.runTests([
+  // Tests if mounting succeeds when invoked from an extension. Note that all
+  // other tests are implemented as apps.
+  function mount() {
+    var onTestSuccess = chrome.test.callbackPass();
+    chrome.fileSystemProvider.mount(
+        {fileSystemId: 'file-system-id', displayName: 'file-system-name'},
+        function() {
+          onTestSuccess();
+        },
+        function(error) {
+          chrome.test.fail();
+        });
+  },
+]);