Allow collection of DOM objects in minor GC cycles.

src/global-handles.h

Index: src/global-handles.h
diff --git a/src/global-handles.h b/src/global-handles.h
index 866317ee139977a199c89ccb07d6be478bed073a..bc7c8af4e472dd66897427fb03f603815a1551c2 100644
--- a/src/global-handles.h
+++ b/src/global-handles.h
@@ -155,6 +155,9 @@ class GlobalHandles {
   // Clear the weakness of a global handle.
   void MarkIndependent(Object** location);
 
+  // Mark the reference to this object externaly unreachable.
+  void MarkExternallyUnreachable(Object** location);
+
   static bool IsIndependent(Object** location);
 
   // Tells whether global handle is near death.