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

Source/platform/heap/HeapTest.cpp

Index: Source/platform/heap/HeapTest.cpp
diff --git a/Source/platform/heap/HeapTest.cpp b/Source/platform/heap/HeapTest.cpp
index 1aafc8a11ed9fe2206c0b5e1b1d344df71588f18..488cc164eca77464018def6da3db5c4a398fdee8 100644
--- a/Source/platform/heap/HeapTest.cpp
+++ b/Source/platform/heap/HeapTest.cpp
@@ -5030,4 +5030,30 @@ TEST(HeapTest, TraceIfNeeded)
     }
 }
 
+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