Add GetCurrentRoutes() to MediaRouter API, ensure dialog has routes at init

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 a550715b3ed37422665cd77bd25faaed7cf579ac..f72bb59cde0bb1c66257b1a4aa7e521b560feb13 100644
--- a/chrome/browser/media/router/media_router.h
+++ b/chrome/browser/media/router/media_router.h
@@ -181,6 +181,10 @@ class MediaRouter : public KeyedService {
   // This will terminate all incognito media routes.
   virtual void OnIncognitoProfileShutdown() = 0;
 
+  // Returns the media routes that currently exist. To get notified whenever
+  // there is a change to the media routes, subclass MediaRoutesObserver.
+  virtual std::vector<MediaRoute> GetCurrentRoutes() const = 0;
+
  private:
   friend class IssuesObserver;
   friend class MediaSinksObserver;