VM: ProcessPendingWeakProperties must call VisitingOldObject(NULL) at the end.

TBR=asiva@google.com

Review URL: https://codereview.chromium.org/2057703002 .
diff --git a/runtime/vm/gc_marker.cc b/runtime/vm/gc_marker.cc
index 5004920..2ac946a 100644
--- a/runtime/vm/gc_marker.cc
+++ b/runtime/vm/gc_marker.cc
@@ -201,7 +201,7 @@
       // Advance to next weak property in the queue.
       cur_weak = reinterpret_cast<RawWeakProperty*>(next_weak);
     }
-
+    VisitingOldObject(NULL);
     return marked;
   }