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

chrome/browser/sync/js/js_event_details.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_event_details.h"
 
 #include "base/json/json_writer.h"
 
 namespace browser_sync {
 
-JsEventDetails::JsEventDetails()
-    : details_(new SharedDictionaryValue()) {}
+JsEventDetails::JsEventDetails() {}
 
-JsEventDetails::JsEventDetails(DictionaryValue* details)
-    : details_(new SharedDictionaryValue(details)) {}
+JsEventDetails::JsEventDetails(DictionaryValue* details) : details_(details) {}
 
 JsEventDetails::~JsEventDetails() {}
 
 const DictionaryValue& JsEventDetails::Get() const {
-  return details_->Get();
+  return details_.Get();
 }
 
 std::string JsEventDetails::ToString() const {
   std::string str;
   base::JSONWriter::Write(&Get(), false, &str);
   return str;
 }
 
 }  // namespace browser_sync