Oilpan: Mark the NodeList hierarchy MainThreadOnly

Source/platform/heap/ThreadState.h

Index: Source/platform/heap/ThreadState.h
diff --git a/Source/platform/heap/ThreadState.h b/Source/platform/heap/ThreadState.h
index 0eb1cb409f052fed3a00ef21cc41170831ab1960..b6405a8fee1d13baf529a6e6061a7341ef851050 100644
--- a/Source/platform/heap/ThreadState.h
+++ b/Source/platform/heap/ThreadState.h
@@ -84,10 +84,15 @@ enum ThreadAffinity {
     MainThreadOnly,
 };
 
+// FIXME: These forward declarations violate dependency rules. Remove them.
+// Ideally we want to provide a USED_IN_MAIN_THREAD_ONLY(T) macro, which
+// indicates that classes in T's hierarchy are used only by the main thread.
 class Node;
-class CSSValue;
+class NodeList;
 
-template<typename T, bool derivesNode = WTF::IsSubclass<typename WTF::RemoveConst<T>::Type, Node>::value> struct DefaultThreadingTrait;
+template<typename T,
+    bool mainThreadOnly = WTF::IsSubclass<typename WTF::RemoveConst<T>::Type, Node>::value
+        || WTF::IsSubclass<typename WTF::RemoveConst<T>::Type, NodeList>::value> struct DefaultThreadingTrait;
 
 template<typename T>
 struct DefaultThreadingTrait<T, false> {