Fix TextTrackList event attribute lookups when in a detached state.

Source/core/html/track/TextTrackList.cpp

 /*
  * Copyright (C) 2011, 2012 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 246 matching lines @@
     return tracks->find(track) != kNotFound;
 }
 
 const AtomicString& TextTrackList::interfaceName() const
 {
     return EventTargetNames::TextTrackList;
 }
 
 ExecutionContext* TextTrackList::executionContext() const
 {
-    ASSERT(m_owner);
-    return m_owner->executionContext();
+    return m_owner ? m_owner->executionContext() : 0;
 }
 
 #if !ENABLE(OILPAN)
 void TextTrackList::clearOwner()
 {
     m_owner = nullptr;
 }
 #endif
 
 void TextTrackList::scheduleTrackEvent(const AtomicString& eventName, PassRefPtrWillBeRawPtr<TextTrack> track)
@@ skipping 53 matching lines @@
     return m_owner;
 }
 
 void TextTrackList::trace(Visitor* visitor)
 {
     visitor->trace(m_owner);
     visitor->trace(m_addTrackTracks);
     visitor->trace(m_elementTracks);
     visitor->trace(m_inbandTracks);
 }