Use the standard downcasting macros for downcasting TextTrack objects.

third_party/WebKit/Source/core/html/track/LoadableTextTrack.h

Index: third_party/WebKit/Source/core/html/track/LoadableTextTrack.h
diff --git a/third_party/WebKit/Source/core/html/track/LoadableTextTrack.h b/third_party/WebKit/Source/core/html/track/LoadableTextTrack.h
index 37ace6915c94a7014127af83cd99faa632d9421f..6884ce8303ca2c0b3a1807be201d170fa79dbae5 100644
--- a/third_party/WebKit/Source/core/html/track/LoadableTextTrack.h
+++ b/third_party/WebKit/Source/core/html/track/LoadableTextTrack.h
@@ -28,7 +28,7 @@
 
 #include "core/html/track/TextTrack.h"
 #include "platform/heap/Handle.h"
-#include "wtf/PassRefPtr.h"
+#include "wtf/Assertions.h"
 
 namespace blink {
 
@@ -60,6 +60,12 @@ class LoadableTextTrack final : public TextTrack {
   Member<HTMLTrackElement> m_trackElement;
 };
 
+DEFINE_TYPE_CASTS(LoadableTextTrack,
+                  TextTrack,
+                  track,
+                  track->trackType() == TextTrack::TrackElement,
+                  track.trackType() == TextTrack::TrackElement);
+
 }  // namespace blink
 
 #endif  // LoadableTextTrack_h