Oilpan: Remove a vtable from the Persistent hierarhcy

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 2e3d01499e2a812f3c027fec85a2568b06ac86f3..f9e68838476560d751bd5429fca06432597186ee 100644
--- a/Source/platform/heap/HeapTest.cpp
+++ b/Source/platform/heap/HeapTest.cpp
@@ -77,6 +77,12 @@ private:
 };
 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) { }