Oilpan: disable conservative GCs during initial GC mixin construction.

Source/core/dom/shadow/ShadowRoot.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.
  *     * Neither the name of Google Inc. nor the names of its
@@ skipping 26 matching lines @@
 #include "core/dom/shadow/ElementShadow.h"
 #include "core/dom/shadow/InsertionPoint.h"
 #include "core/dom/shadow/ShadowRootRareData.h"
 #include "core/editing/markup.h"
 #include "core/html/HTMLShadowElement.h"
 #include "public/platform/Platform.h"
 
 namespace blink {
 
 struct SameSizeAsShadowRoot : public DocumentFragment, public TreeScope, public DoublyLinkedListNode<ShadowRoot> {
+#if ENABLE(OILPAN)
+    char emptyClassFieldsDueToGCMixinMarker[1];
+#endif
     void* pointers[3];
     unsigned countersAndFlags[1];
 };
 
 static_assert(sizeof(ShadowRoot) == sizeof(SameSizeAsShadowRoot), "ShadowRoot should stay small");
 
 ShadowRoot::ShadowRoot(Document& document, ShadowRootType type)
     : DocumentFragment(0, CreateShadowRoot)
     , TreeScope(*this, document)
     , m_prev(nullptr)
@@ skipping 265 matching lines @@
 DEFINE_TRACE(ShadowRoot)
 {
     visitor->trace(m_prev);
     visitor->trace(m_next);
     visitor->trace(m_shadowRootRareData);
     TreeScope::trace(visitor);
     DocumentFragment::trace(visitor);
 }
 
 }