[Media Router] Conditionally enable mDNS on Windows.

chrome/browser/media/router/media_router.h

Index: chrome/browser/media/router/media_router.h
diff --git a/chrome/browser/media/router/media_router.h b/chrome/browser/media/router/media_router.h
index d932d9330a0d0469a057d29fe4bddcca1d4a315d..3602a1f392ab97d797c4562e299fd2a2e37358f6 100644
--- a/chrome/browser/media/router/media_router.h
+++ b/chrome/browser/media/router/media_router.h
@@ -154,6 +154,11 @@ class MediaRouter : public KeyedService {
   // Clears the issue with the id |issue_id|.
   virtual void ClearIssue(const Issue::Id& issue_id) = 0;
 
+  // Notifies the Media Router that the user has taken an action involving the
+  // Media Router. This can be used to perform any initialization that is not
+  // approriate to be done at construction.
+  virtual void OnUserInteraction() = 0;

[mark a. foltz, 2016/03/29 00:48:20]

Sorry to bikeshed, but often user initiated actions on the Web are called "user
gestures", so maybe OnUserGesture().  I don't feel strongly though.

+
   // Adds |callback| to listen for state changes for presentation connected to
   // |route_id|. The returned Subscription object is owned by the caller.
   // |callback| will be invoked whenever there are state changes, until the