[Sync] Rework SharedValue<T> into Immutable<T>

chrome/browser/sync/js/js_arg_list.cc

 // Copyright (c) 2011 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 "chrome/browser/sync/js/js_arg_list.h"
 
 #include "base/json/json_writer.h"
 
 namespace browser_sync {
 
-JsArgList::JsArgList() : args_(new SharedListValue()) {}
+JsArgList::JsArgList() {}
 
-JsArgList::JsArgList(ListValue* args)
-    : args_(new SharedListValue(args)) {}
+JsArgList::JsArgList(ListValue* args) : args_(args) {}
 
 JsArgList::~JsArgList() {}
 
 const ListValue& JsArgList::Get() const {
-  return args_->Get();
+  return args_.Get();
 }
 
 std::string JsArgList::ToString() const {
   std::string str;
   base::JSONWriter::Write(&Get(), false, &str);
   return str;
 }
 
 }  // namespace browser_sync