Profile mojom: change some fields to nullable according to documentation.

mojo/services/public/interfaces/profile/profile_service.mojom

-// 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.
-
-module mojo {
-
-// Provides access to system information.
-interface ProfileService {
-
-  enum PathKey {
-    DIR_TEMP  // temporary directory
-  };
-
-  // Returns a path to a special directory or file.  If you ask for a directory
-  // it is guaranteed to NOT have a path separator at the end. For example,
-  // "c:\windows\temp" Returns |null| if the directory of path doesn't exist.
-  // On windows and Mac OS X and Chrome OS, path will be utf-8 encoded, on all
-  // other platform, the encoding is not specified and may depend on the file
-  // system.
-  GetPath(PathKey key) => (string path);
-};
-
-}