Remove Chromecast extension support from cast system tray menu.

ash/common/system/chromeos/cast/tray_cast.h

Index: ash/common/system/chromeos/cast/tray_cast.h
diff --git a/ash/common/system/chromeos/cast/tray_cast.h b/ash/common/system/chromeos/cast/tray_cast.h
index e7d7ba1a05dd5f30ad0cf4c4a491b073466ccb72..270114a9911abcd5c78245aba150c74a00824599 100644
--- a/ash/common/system/chromeos/cast/tray_cast.h
+++ b/ash/common/system/chromeos/cast/tray_cast.h
@@ -51,9 +51,6 @@ class ASH_EXPORT TrayCast : public SystemTrayItem,
   void OnDevicesUpdated(
       const CastConfigDelegate::ReceiversAndActivities& devices) override;
 
-  // Returns true if the cast extension was detected.
-  bool HasCastExtension();
-
   // This makes sure that the current view displayed in the tray is the correct
   // one, depending on if we are currently casting. If we're casting, then a
   // view with a stop button is displayed; otherwise, a view that links to a