[Media Router] Add CastMediaSinkService

chrome/browser/media/router/discovery/mdns/mock_dns_sd_registry.cc

 // Copyright 2017 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/media/router/discovery/mdns/mock_dns_sd_registry.h"
 
 namespace media_router {
 
 MockDnsSdRegistry::MockDnsSdRegistry(DnsSdRegistry::DnsSdObserver* observer)
-    : observer_(observer) {}
+    : DnsSdRegistry(nullptr), observer_(observer) {}
 
 MockDnsSdRegistry::~MockDnsSdRegistry() {}
 
 void MockDnsSdRegistry::DispatchMDnsEvent(const std::string& service_type,
                                           const DnsSdServiceList& services) {
   observer_->OnDnsSdEvent(service_type, services);
 }
 
 }  // namespace media_router