MediaRouterAction: Only observe Media Routes when there is a local route.

chrome/browser/media/router/media_router.gypi

 # Copyright (c) 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.
 
 {
   'variables': {
     # File lists shared with GN build.
     'media_router_sources': [
       'create_presentation_session_request.cc',
       'create_presentation_session_request.h',
       'issue.cc',
       'issue.h',
       'issue_manager.cc',
       'issue_manager.h',
       'issues_observer.h',
       'issues_observer.cc',
+      'local_media_routes_observer.cc',
+      'local_media_routes_observer.h',
       'media_route.cc',
       'media_route.h',
       'media_router.h',
       'media_router_dialog_controller.cc',
       'media_router_dialog_controller.h',
       'media_router_factory.cc',
       'media_router_factory.h',
       'media_routes_observer.cc',
       'media_routes_observer.h',
       'media_sink.cc',
@@ skipping 25 matching lines @@
       'media_router_mojo_test.h',
       'mock_media_router.cc',
       'mock_media_router.h',
       'mock_screen_availability_listener.cc',
       'mock_screen_availability_listener.h',
       'test_helper.cc',
       'test_helper.h',
     ],
   },
 }