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 4f317179f9196bb2046402f6d202f12042dbe51d..e6584c567bedc7973b85708ab87869cec7712b0d 100644
--- a/chrome/browser/media/router/media_router.h
+++ b/chrome/browser/media/router/media_router.h
@@ -183,6 +183,10 @@ 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;