[Media Router] Conditionally enable mDNS on Windows.

chrome/browser/media/router/media_router_metrics.h

Index: chrome/browser/media/router/media_router_metrics.h
diff --git a/chrome/browser/media/router/media_router_metrics.h b/chrome/browser/media/router/media_router_metrics.h
index 9eda482a2852af5a9284def8f04ac3cc8a351e37..a7c9e17f0603e27d960afbcd41452291c8dbc5a4 100644
--- a/chrome/browser/media/router/media_router_metrics.h
+++ b/chrome/browser/media/router/media_router_metrics.h
@@ -76,9 +76,10 @@ enum class MediaRouteProviderWakeReason {
   CONNECTION_ERROR = 12,
   REGISTER_MEDIA_ROUTE_PROVIDER = 13,
   CONNECT_ROUTE_BY_ROUTE_ID = 14,
+  ENABLE_MDNS_DISCOVERY = 15,
 
   // NOTE: Add entries only immediately above this line.
-  TOTAL_COUNT = 15
+  TOTAL_COUNT = 16
 };
 
 // The install status of the Media Router component extension.