[Media Router] Conditionally enable mDNS on Windows.

chrome/browser/media/router/media_router.mojom

Index: chrome/browser/media/router/media_router.mojom
diff --git a/chrome/browser/media/router/media_router.mojom b/chrome/browser/media/router/media_router.mojom
index 6c28dd7bc129b6ecc872bd120e88fd0104865fd2..7ebcc813f7f83e4b0b5332b44fc8e584356eb2be 100644
--- a/chrome/browser/media/router/media_router.mojom
+++ b/chrome/browser/media/router/media_router.mojom
@@ -263,6 +263,11 @@ interface MediaRouteProvider {
   // |route_id| has been closed (via .close(), garbage collection or
   // navigation).
   DetachRoute(string route_id);
+
+  // Enables mDNS discovery. No-op if mDNS discovery is already enabled.
+  // Calling this will trigger a firewall prompt on Windows if there is not
+  // already a firewall rule for mDNS.
+  EnableMdnsDiscovery();
 };
 
 // Interface for a service which observes state changes across media