[mDns] Make DnsSdRegistry a leaky singleton

chrome/browser/media/router/BUILD.gn

 # Copyright 2015 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.
 #
 
 import("//testing/test.gni")
 
 static_library("router") {
   deps = [
     "//base",
@@ skipping 91 matching lines @@
     "test_helper.h",
   ]
 
   if (!is_android) {
     deps += [
       "//chrome/common/media_router/mojo:media_router",
       "//chrome/common/media_router/mojo:media_router_test_interfaces",
       "//extensions/common",
     ]
     sources += [
+      "discovery/mdns/mock_dns_sd_registry.cc",
+      "discovery/mdns/mock_dns_sd_registry.h",
       "mojo/media_router_mojo_test.cc",
       "mojo/media_router_mojo_test.h",
     ]
   }
 }