Oilpan: fix tracing of un-initialized part objects during conservative GCs.

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 5012 matching lines @@
 
     {
         TraceIfNeededTester<HeapVector<Member<SimpleObject> > >* m_vec = TraceIfNeededTester<HeapVector<Member<SimpleObject> > >::create();
         m_vec->obj().append(SimpleObject::create());
         visitor.reset();
         m_vec->trace(&visitor);
         EXPECT_EQ(2u, visitor.count());
     }
 }
 
+static bool allocateAndReturnBool()
+{
+    Heap::collectGarbage(ThreadState::HeapPointersOnStack);
+    return true;
+}
+
+class PartObjectWithVirtualMethod {
+public:
+    virtual void trace(Visitor*) { }
+};
+
+class ObjectWithVirtualPartObject : public GarbageCollected<ObjectWithVirtualPartObject> {
+public:
+    ObjectWithVirtualPartObject() : m_dummy(allocateAndReturnBool()) { }
+    void trace(Visitor* visitor) { visitor->trace(m_part); }
+private:
+    bool m_dummy;
+    PartObjectWithVirtualMethod m_part;
+};
+
+TEST(HeapTest, PartObjectWithVirtualMethod)
+{
+    ObjectWithVirtualPartObject* object = new ObjectWithVirtualPartObject();
+    EXPECT_TRUE(object);
+}
+
 } // namespace blink