Clean up GC callbacks to be virtual methods

Change-Id: Ia08034a4e5931c4fcb329c3bd3c4b1f301135735
diff --git a/runtime/java_vm_ext.h b/runtime/java_vm_ext.h
index 694a545..97fbbc5 100644
--- a/runtime/java_vm_ext.h
+++ b/runtime/java_vm_ext.h
@@ -120,7 +120,7 @@
 
   void DeleteWeakGlobalRef(Thread* self, jweak obj);
 
-  void SweepJniWeakGlobals(IsMarkedCallback* callback, void* arg)
+  void SweepJniWeakGlobals(IsMarkedVisitor* visitor)
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
   mirror::Object* DecodeGlobal(Thread* self, IndirectRef ref)