[Media Router] Wire through a new MRPM call to update media sinks.

chrome/browser/media/router/mojo/media_router.mojom

Index: chrome/browser/media/router/mojo/media_router.mojom
diff --git a/chrome/browser/media/router/mojo/media_router.mojom b/chrome/browser/media/router/mojo/media_router.mojom
index 683904122489d906d2ffbca2765f76648c824c13..07b918dc744fac7f764d199971b1fc3f4a936436 100644
--- a/chrome/browser/media/router/mojo/media_router.mojom
+++ b/chrome/browser/media/router/mojo/media_router.mojom
@@ -286,6 +286,10 @@ interface MediaRouteProvider {
   // Calling this will trigger a firewall prompt on Windows if there is not
   // already a firewall rule for mDNS.
   EnableMdnsDiscovery();
+
+  // Starts discovery. No-op for MRPs which have already registered discovery
+  // listeners.
+  StartDiscovery();

[mark a. foltz, 2016/04/07 22:45:00]

Nit on naming:

The MRPM may already be running discovery.

Perhaps UpdateMediaSinks() since the function is to potentially send a new sink
list via MediaSinkObservers.

I think this should take a |media_source|, like Start/StopObservingMediaSinks.

Rationale:  The model is that the MRPM is responsible for relaying sinks that
support a specific set of media sources.  This allows the MRPM to be smart about
how to update, and gives the caller more control over which observers get
updates.

If there is a use case for "update all sources," perhaps the caller could pass
an empty |media_source|.   But only if necessary.  

[amp, 2016/04/09 00:02:12]

Done.  I like the UpdateMediaSinks name better.  For the source, I pass in the
Desktop source (since that works in all mirroring cases).

 };
 
 // Interface for a service which observes state changes across media