Oilpan: Trace VTTParser::m_document.

Source/core/html/track/vtt/VTTParser.cpp

 /*
  * Copyright (C) 2011 Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 543 matching lines @@
             m_currentNode->parserAppendChild(ProcessingInstruction::create(document, "timestamp", charactersString));
         break;
     }
     default:
         break;
     }
 }
 
 void VTTParser::trace(Visitor* visitor)
 {
+    visitor->trace(m_document);
     visitor->trace(m_cueList);
     visitor->trace(m_regionList);
 }
 
 }