Use TextTrackMode and TextTrackKind enums in TextTrack IDL

Source/core/html/track/TextTrack.cpp

Index: Source/core/html/track/TextTrack.cpp
diff --git a/Source/core/html/track/TextTrack.cpp b/Source/core/html/track/TextTrack.cpp
index bf17e6cda739657f7c0bbc0d98c1c55df3f2d681..12626dea6144e2789a3b534a6fb01750b5a2d700 100644
--- a/Source/core/html/track/TextTrack.cpp
+++ b/Source/core/html/track/TextTrack.cpp
@@ -162,11 +162,10 @@ void TextTrack::setKind(const AtomicString& kind)
 
 void TextTrack::setMode(const AtomicString& mode)
 {
+    ASSERT(mode == disabledKeyword() || mode == hiddenKeyword() || mode == showingKeyword());
+
     // On setting, if the new value isn't equal to what the attribute would currently
     // return, the new value must be processed as follows ...
-    if (mode != disabledKeyword() && mode != hiddenKeyword() && mode != showingKeyword())
-        return;
-
     if (m_mode == mode)
         return;