shill: Profile: change ctor arg type from string to FilePath

The |user_storage_directory| argument to the Profile ctor
is stored inside the Profile object as a FilePath, and the
DefaultProfile ctor takes its path argument as a FilePath.

Both of these facts argue for |user_storage_directory| to
be passed in as a FilePath, rather than a string.

Make it so.

Bug: None
TEST=unit tests

Change-Id: I6094aeb44de96ccd0312c5311bdd195ee64cf510
diff --git a/manager.h b/manager.h
index e5c7ea9..d47273c 100644
--- a/manager.h
+++ b/manager.h
@@ -743,7 +743,7 @@
   EventDispatcher* dispatcher_;
   const base::FilePath run_path_;
   const base::FilePath storage_path_;
-  const std::string user_storage_path_;
+  const base::FilePath user_storage_path_;
   base::FilePath user_profile_list_path_;  // Changed in tests.
   std::unique_ptr<ManagerAdaptorInterface> adaptor_;
   DeviceInfo device_info_;