[Media Router] Add some SetForTest() functions to DialMediaSinkService and update unit tests

chrome/browser/media/router/discovery/dial/dial_registry.h

Index: chrome/browser/media/router/discovery/dial/dial_registry.h
diff --git a/chrome/browser/media/router/discovery/dial/dial_registry.h b/chrome/browser/media/router/discovery/dial/dial_registry.h
index 93afe5418c9243ce5f4d8eb8ec7232b7b4f4f14a..8b7c7083b25af213f2d68ed9ea1b7f591f53a704 100644
--- a/chrome/browser/media/router/discovery/dial/dial_registry.h
+++ b/chrome/browser/media/router/discovery/dial/dial_registry.h
@@ -76,8 +76,8 @@ class DialRegistry : public DialService::Observer,
 
   // Starts and stops periodic discovery.  Periodic discovery is done when there
   // are registered event listeners.
-  void StartPeriodicDiscovery();
-  void StopPeriodicDiscovery();
+  virtual void StartPeriodicDiscovery();
+  virtual void StopPeriodicDiscovery();
 
   // Returns the URL of the device description for the device identified by
   // |label|, or an empty GURL if no such device exists.