Break two classes defined in json_value_serializer.cc, .h into separate files.

chrome/common/json_value_serializer_perftest.cc

Index: chrome/common/json_value_serializer_perftest.cc
===================================================================
--- chrome/common/json_value_serializer_perftest.cc	(revision 123668)
+++ chrome/common/json_value_serializer_perftest.cc	(working copy)
@@ -1,11 +1,11 @@
-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// 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.
 
 #include <vector>
 
 #include "base/file_util.h"
-#include "base/json/json_value_serializer.h"
+#include "base/json/json_string_value_serializer.h"
 #include "base/path_service.h"
 #include "base/perftimer.h"
 #include "base/string_util.h"