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

chrome/browser/prefs/pref_model_associator.cc

Index: chrome/browser/prefs/pref_model_associator.cc
===================================================================
--- chrome/browser/prefs/pref_model_associator.cc	(revision 123668)
+++ chrome/browser/prefs/pref_model_associator.cc	(working copy)
@@ -1,4 +1,4 @@
-// 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.
 
@@ -6,7 +6,7 @@
 
 #include "base/auto_reset.h"
 #include "base/json/json_reader.h"
-#include "base/json/json_value_serializer.h"
+#include "base/json/json_string_value_serializer.h"
 #include "base/location.h"
 #include "base/logging.h"
 #include "base/utf_string_conversions.h"