Oilpan: Trace DocumentVisibilityObserver::m_document.

Source/core/html/HTMLCanvasElement.cpp

 /*
  * Copyright (C) 2004, 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. 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.
@@ skipping 523 matching lines @@
     if (m_context && m_context->is2d()) {
         CanvasRenderingContext2D* context2d = toCanvasRenderingContext2D(m_context.get());
         context2d->loseContext();
     }
 }
 
 void HTMLCanvasElement::trace(Visitor* visitor)
 {
     visitor->trace(m_observers);
     visitor->trace(m_context);
+    DocumentVisibilityObserver::trace(visitor);
     HTMLElement::trace(visitor);
 }
 
 void HTMLCanvasElement::updateExternallyAllocatedMemory() const
 {
     int bufferCount = 0;
     if (m_imageBuffer)
         bufferCount++;
     if (is3D())
         bufferCount += 2;
@@ skipping 147 matching lines @@
 {
     return !originClean();
 }
 
 FloatSize HTMLCanvasElement::sourceSize() const
 {
     return FloatSize(width(), height());
 }
 
 }