Add DiscoveryNetworkMonitor implementation

chrome/test/BUILD.gn

Index: chrome/test/BUILD.gn
diff --git a/chrome/test/BUILD.gn b/chrome/test/BUILD.gn
index 9072029d2cfffc7578cb9b50312c2b9e6e699022..75afdc832cfb8459efdd4a97eb23f99e1bdb2585 100644
--- a/chrome/test/BUILD.gn
+++ b/chrome/test/BUILD.gn
@@ -4380,6 +4380,8 @@ test("unit_tests") {
       "../browser/media/cast_remoting_connector_unittest.cc",
       "../browser/media/router/browser_presentation_connection_proxy_unittest.cc",
       "../browser/media/router/create_presentation_connection_request_unittest.cc",
+      "../browser/media/router/discovery/discovery_network_list_unittest.cc",
+      "../browser/media/router/discovery/discovery_network_monitor_unittest.cc",
       "../browser/media/router/issue_manager_unittest.cc",
       "../browser/media/router/issue_unittest.cc",
       "../browser/media/router/media_route_unittest.cc",
@@ -4510,9 +4512,8 @@ test("unit_tests") {
       "//third_party/ocmock",
     ]
 
-    sources += [
-      "../browser/ui/cocoa/applescript/apple_event_util_unittest.mm",
-    ]
+    sources +=
+        [ "../browser/ui/cocoa/applescript/apple_event_util_unittest.mm" ]
 
     if (mac_views_browser) {
       # TODO(tapted): Add chrome_unit_tests_views_non_mac_sources.