Oilpan: Ensure that types definitions are known for traced weak members.

Source/platform/heap/Visitor.h

Index: Source/platform/heap/Visitor.h
diff --git a/Source/platform/heap/Visitor.h b/Source/platform/heap/Visitor.h
index 56cfbef708c3273e745fb264ef1d08e098b867f4..14029fc2374ec8a66ddc1cb3e3c9dd9344ac5590 100644
--- a/Source/platform/heap/Visitor.h
+++ b/Source/platform/heap/Visitor.h
@@ -259,6 +259,8 @@ public:
     template<typename T>
     void trace(const WeakMember<T>& t)
     {
+        // Check that we actually know the definition of T when tracing.
+        COMPILE_ASSERT(sizeof(T), WeNeedToKnowTheDefinitionOfTheTypeWeAreTracing);
         registerWeakCell(const_cast<WeakMember<T>&>(t).cell());
     }