Implement PersistentValueMap, a map that stores UniquePersistent values.

src/global-handles.h

Index: src/global-handles.h
diff --git a/src/global-handles.h b/src/global-handles.h
index a40645199c61b1d24a766f18cb5d8847468d0631..e698c82218ca75fb883d4a60c69747fb2310d662 100644
--- a/src/global-handles.h
+++ b/src/global-handles.h
@@ -161,7 +161,7 @@ class GlobalHandles {
   }
 
   // Clear the weakness of a global handle.
-  static void ClearWeakness(Object** location);
+  static void* ClearWeakness(Object** location);
 
   // Clear the weakness of a global handle.
   static void MarkIndependent(Object** location);