Revise user-data-dir switch handling; avoid crashes.

chrome/browser/user_data_dir_extractor.h

 // Copyright (c) 2013 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.
 
 #ifndef CHROME_BROWSER_USER_DATA_DIR_EXTRACTOR_H_
 #define CHROME_BROWSER_USER_DATA_DIR_EXTRACTOR_H_
 
+#include "base/callback.h"
+
 namespace base {
 class FilePath;
 }
 
 namespace content{
 struct MainFunctionParams;
 }
 
 namespace chrome {
 
+typedef base::Callback<base::FilePath()> GetUserDataDirCallback;
+
+// Tests may install a custom GetUserDataDir() callback to override behavior.
+void InstallCustomGetUserDataDirCallbackForTest(
+    GetUserDataDirCallback* callback);
+
 // Returns the user data dir. Must be called prior to InitializeLocalState().
 base::FilePath GetUserDataDir(const content::MainFunctionParams& parameters);
 
 }  // namespace chrome
 
 #endif  // CHROME_BROWSER_USER_DATA_DIR_EXTRACTOR_H_