[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 bb9beabbceed922b65120ceecf16ac6a6382eee8..683904122489d906d2ffbca2765f76648c824c13 100644
--- a/chrome/browser/media/router/media_router.mojom
+++ b/chrome/browser/media/router/media_router.mojom
@@ -281,6 +281,11 @@ 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