browser: Refactor the way to show the user data dir dialog.

chrome/browser/ui/user_data_dir_dialog.h

+// 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.
+
+#ifndef CHROME_BROWSER_UI_USER_DATA_DIR_DIALOG_H_
+#define CHROME_BROWSER_UI_USER_DATA_DIR_DIALOG_H_
+#pragma once
+
+class FilePath;
+
+namespace browser {
+
+// Shows a user data directory picker dialog. The method blocks while the dialog
+// is showing. If the user picks a directory, this method returns the chosen
+// directory. |user_data_dir| is the value of the directory we were not able to
+// use.
+FilePath ShowUserDataDirDialog(const FilePath& user_data_dir);
+
+}  // namespace browser
+
+#endif  // CHROME_BROWSER_UI_USER_DATA_DIR_DIALOG_H_