Oilpan: Remove a vtable from the Persistent hierarhcy

Source/platform/heap/HeapTest.cpp

 /*
  * Copyright (C) 2013 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 59 matching lines @@
     unsigned hash() { return IntHash<int>::hash(m_x); }
 
     IntWrapper(int x) : m_x(x) { }
 
 private:
     IntWrapper();
     int m_x;
 };
 static_assert(WTF::NeedsTracing<IntWrapper>::value, "NeedsTracing macro failed to recognize trace method.");
 
+struct SameSizeAsPersistent {
+    void* m_pointer[4];
+};
+
+static_assert(sizeof(Persistent<IntWrapper>) <= sizeof(SameSizeAsPersistent), "Persistent handle should stay small");
+
 class ThreadMarker {
 public:
     ThreadMarker() : m_creatingThread(reinterpret_cast<ThreadState*>(0)), m_num(0) { }
     ThreadMarker(unsigned i) : m_creatingThread(ThreadState::current()), m_num(i) { }
     ThreadMarker(WTF::HashTableDeletedValueType deleted) : m_creatingThread(reinterpret_cast<ThreadState*>(-1)), m_num(0) { }
     ~ThreadMarker()
     {
         EXPECT_TRUE((m_creatingThread == ThreadState::current())
             || (m_creatingThread == reinterpret_cast<ThreadState*>(0))
             || (m_creatingThread == reinterpret_cast<ThreadState*>(-1)));
@@ skipping 6029 matching lines @@
 {
     Persistent<ClassWithMember> object = ClassWithMember::create();
     EXPECT_EQ(0, object->traceCount());
     TestMixinAllocatingObject* mixin = TestMixinAllocatingObject::create(object.get());
     EXPECT_TRUE(mixin);
     EXPECT_GT(object->traceCount(), 0);
     EXPECT_GT(mixin->traceCount(), 0);
 }
 
 } // namespace blink