Replace redundant <track> runtime checks with asserts

Source/core/html/HTMLMediaElement.cpp

 /*
  * Copyright (C) 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 2517 matching lines @@
     for (int i = m_textTracks->length() - 1; i >= 0; --i) {
         TextTrack* track = m_textTracks->item(i);
 
         if (track->trackType() == TextTrack::InBand)
             removeTrack(track);
     }
 }
 
 PassRefPtr<TextTrack> HTMLMediaElement::addTextTrack(const String& kind, const String& label, const String& language, ExceptionState& es)
 {
-    if (!RuntimeEnabledFeatures::videoTrackEnabled())
-        return 0;
+    ASSERT(RuntimeEnabledFeatures::videoTrackEnabled());
 
     // 4.8.10.12.4 Text track API
     // The addTextTrack(kind, label, language) method of media elements, when invoked, must run the following steps:
 
     // 1. If kind is not one of the following strings, then throw a SyntaxError exception and abort these steps
     if (!TextTrack::isValidKindKeyword(kind)) {
         es.throwUninformativeAndGenericDOMException(SyntaxError);
         return 0;
     }
 
@@ skipping 15 matching lines @@
     textTrack->setReadinessState(TextTrack::Loaded);
 
     // ... its text track mode to the text track hidden mode, and its text track list of cues to an empty list ...
     textTrack->setMode(TextTrack::hiddenKeyword());
 
     return textTrack.release();
 }
 
 TextTrackList* HTMLMediaElement::textTracks()
 {
-    if (!RuntimeEnabledFeatures::videoTrackEnabled())
-        return 0;
+    ASSERT(RuntimeEnabledFeatures::videoTrackEnabled());
 
     if (!m_textTracks)
         m_textTracks = TextTrackList::create(this);
 
     return m_textTracks.get();
 }
 
 void HTMLMediaElement::didAddTrack(HTMLTrackElement* trackElement)
 {
     ASSERT(trackElement->hasTagName(trackTag));
@@ skipping 1285 matching lines @@
 {
     scheduleLayerUpdate();
 }
 
 bool HTMLMediaElement::isInteractiveContent() const
 {
     return fastHasAttribute(controlsAttr);
 }
 
 }