MediaSinkService defines an interface which can be implemented by DIAL / Cast / future protocols to produce MediaSinks from device discovery/resolution. The service is also responsible for answering MediaSink queries.

The interface will look like the following (copied from design doc):

class MediaSinkService {
 public:
  // Callback used for |Start()|.
  // Arg 0: |this|
  // Arg 1: Sinks discovered and resolved by the service.
  using OnSinksDiscoveredCallback = base::Callback<void(
      const const MediaSinkService*, const std::vector<MediaSink>&)>;

  // Starts sink discovery. No-ops if already started.
  // Sinks discovered and resolved are continuously passed to 
  // |callback|.
  void Start(const OnSinksDiscoveredCallback& callback) = 0;

  // Adds a sink query to observe for MediaSink updates.
  // Only one observer can be added for a given MediaSource.
  // Start() must be called first. This class does not take
  // ownership of |observer|.
  void AddSinkQuery(MediaSinksObserver* observer) = 0;

  // Removes a sink query and stop observing for MediaSink updates.
  void RemoveSinkQuery(MediaSinksObserver* observer) = 0;
};

Each protocol (DIAL and Cast for now) will implement their own MediaSinkService, which can then be independently registered with MediaRouter.