[Media Router] Create MediaSinkServiceBase and make DialMediaSinkService inherit from it

chrome/common/media_router/discovery/media_sink_service.h

Index: chrome/common/media_router/discovery/media_sink_service.h
diff --git a/chrome/common/media_router/discovery/media_sink_service.h b/chrome/common/media_router/discovery/media_sink_service.h
index 5e2f0bcd48f44b591949db9d916e0167af88fc89..5f88cf916e743ee61fabcef3d5faa538de2d51ce 100644
--- a/chrome/common/media_router/discovery/media_sink_service.h
+++ b/chrome/common/media_router/discovery/media_sink_service.h
@@ -18,7 +18,7 @@ namespace media_router {
 // MediaSinks. Often these are remote devices, like Chromecast. In addition, the
 // service is capable of answering MediaSink queries using the sinks that it
 // generated.
-// This class is not thread safe. All methods must be called from the IO thread.
+// This class is not thread safe. All methods must be called from the UI thread.
 class MediaSinkService {
  public:
   // Callback to be invoked when this class finishes sink discovering.
@@ -36,6 +36,9 @@ class MediaSinkService {
   // |callback|.
   virtual void Start() = 0;
 
+  // Stops sink discovery. No-ops if already stopped.
+  virtual void Stop() = 0;
+
  protected:
   OnSinksDiscoveredCallback sink_discovery_callback_;