Oilpan:Allow custom handling of classes that contain weak pointers that are embedded in collections.

Source/wtf/ListHashSet.h

Index: Source/wtf/ListHashSet.h
diff --git a/Source/wtf/ListHashSet.h b/Source/wtf/ListHashSet.h
index 047aa2478cd0b58fc0f16af7cb2964eda09bebe1..d7da0924813e89c0af32ca0b9bc2d277e13308c9 100644
--- a/Source/wtf/ListHashSet.h
+++ b/Source/wtf/ListHashSet.h
@@ -996,6 +996,7 @@ namespace WTF {
     template<typename T, size_t inlineCapacity, typename U, typename V>
     void ListHashSet<T, inlineCapacity, U, V>::trace(typename Allocator::Visitor* visitor)
     {
+        COMPILE_ASSERT(!HashTraits<T>::isWeak, ListHashSetDoesNotSupportWeakness);
         // This marks all the nodes and their contents live that can be
         // accessed through the HashTable.
         m_impl.trace(visitor);