[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..eaf09abd2147411db1ce53a952fe968a95310b24 100644
--- a/chrome/browser/media/router/media_router.h
+++ b/chrome/browser/media/router/media_router.h
@@ -154,6 +154,13 @@ class MediaRouter : public KeyedService {
   // Clears the issue with the id |issue_id|.
   virtual void ClearIssue(const Issue::Id& issue_id) = 0;
 
+  // Returns |true| if mDNS discovery is enabled.
+  virtual bool should_enable_mdns_discovery() const = 0;

[mark a. foltz, 2016/03/25 23:14:16]

But the method sounds like mdns will be enabled in the future?  Consider naming
this is_mdns_enabled()

[btolsch, 2016/03/26 10:53:58]

Due to other changes, it turns out this is no longer needed so I removed it.
This also slightly cleans up the Android and test code that was forced to
"implement" these methods.

+
+  // Enables mDNS discovery on Media Route Provider Manager.
+  // |should_enable_mdns_discovery| should also return |true| after this call.
+  virtual void EnsureMdnsDiscoveryEnabled() = 0;

[mark a. foltz, 2016/03/25 23:14:16]

I don't feel great about exposing this API at the MediaRouter level.  It's
really an API to trigger a specific codepath on a specific platform (Windows) in
response to something happening in the UI.

Maybe there's a slightly more general way to think about this flow, like naming
it OnMediaSinkSelection() or OnUserInteraction(), which is implemented by
enabling mDNS if necessary.  

I don't want to derail this into a bikeshed, but consider adding a TODO() to
make this API more generic if we find other use cases that follow the same
pattern.

[btolsch, 2016/03/26 10:53:58]

I think that since should_enable_mdns_discovery() was removed it is simple to
rename this to OnUserInteraction() now.

+
   // 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