Make base compile with no "using base::FilePath".

base/json/json_file_value_serializer.cc

Index: base/json/json_file_value_serializer.cc
===================================================================
--- base/json/json_file_value_serializer.cc	(revision 182300)
+++ base/json/json_file_value_serializer.cc	(working copy)
@@ -8,6 +8,8 @@
 #include "base/json/json_string_value_serializer.h"
 #include "base/logging.h"
 
+using base::FilePath;
+
 const char* JSONFileValueSerializer::kAccessDenied = "Access denied.";
 const char* JSONFileValueSerializer::kCannotReadFile = "Can't read file.";
 const char* JSONFileValueSerializer::kFileLocked = "File locked.";