[oilpan] The Node hierarchy should have correct accept method chains

Source/core/html/PluginDocument.cpp

 /*
  * Copyright (C) 2006, 2008 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 167 matching lines @@
     // PluginDocument::cancelManualPluginLoad should only be called once, but there are issues
     // with how many times we call beforeload on object elements. <rdar://problem/8441094>.
     if (!shouldLoadPluginManually())
         return;
 
     DocumentLoader* documentLoader = frame()->loader()->activeDocumentLoader();
     documentLoader->cancelMainResourceLoad(frame()->loader()->cancelledError(documentLoader->request()));
     setShouldLoadPluginManually(false);
 }
 
+void PluginDocument::acceptHeapVisitor(Visitor* visitor) const
+{
+    HTMLDocument::acceptHeapVisitor(visitor);
 }
+
+}