Add an API for enumerating persistent handles

src/global-handles.h

Index: src/global-handles.h
diff --git a/src/global-handles.h b/src/global-handles.h
index ddf5fe29c383fcd87b5d9d8b7d576d662ae11e4b..866317ee139977a199c89ccb07d6be478bed073a 100644
--- a/src/global-handles.h
+++ b/src/global-handles.h
@@ -131,6 +131,7 @@ class GlobalHandles {
                 WeakReferenceCallback callback);
 
   static void SetWrapperClassId(Object** location, uint16_t class_id);
+  static uint16_t GetWrapperClassId(Object** location);
 
   // Returns the current number of weak handles.
   int NumberOfWeakHandles() { return number_of_weak_handles_; }
@@ -154,6 +155,8 @@ class GlobalHandles {
   // Clear the weakness of a global handle.
   void MarkIndependent(Object** location);
 
+  static bool IsIndependent(Object** location);
+
   // Tells whether global handle is near death.
   static bool IsNearDeath(Object** location);