Allow creating non-thread-local Persistents that can be passed between threads.

Source/heap/ThreadState.h

Index: Source/heap/ThreadState.h
diff --git a/Source/heap/ThreadState.h b/Source/heap/ThreadState.h
index f240c2325cbb23766085dfd9e10586ca8beecf3d..8b22a5bca21381ccef3343863906fac056646df4 100644
--- a/Source/heap/ThreadState.h
+++ b/Source/heap/ThreadState.h
@@ -38,6 +38,7 @@
 #include "wtf/PassOwnPtr.h"
 #include "wtf/ThreadSpecific.h"
 #include "wtf/Threading.h"
+#include "wtf/ThreadingPrimitives.h"
 #include "wtf/Vector.h"
 
 namespace WebCore {
@@ -453,6 +454,11 @@ public:
     // List of persistent roots allocated on the given thread.
     PersistentNode* roots() const { return m_persistents; }
 
+    // List of global persistent roots not owned by any particular thread.
+    // globalRootsMutex must be acquired before any modifications.
+    static PersistentNode* globalRoots();
+    static Mutex& globalRootsMutex();
+
     // Visit local thread stack and trace all pointers conservatively.
     void visitStack(Visitor*);