[mDns] Make DnsSdRegistry a leaky singleton

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) {}
+
+MockDnsSdRegistry::~MockDnsSdRegistry() {}
+
+void MockDnsSdRegistry::DispatchMDnsEvent(const std::string& service_type,
+                                          const DnsSdServiceList& services) {
+  observer_->OnDnsSdEvent(service_type, services);
+}
+
+}  // namespace media_router