Fix portable build shadow frame.

Change-Id: I78cbf09dbe2bcfc0c3b8bf40f804ae65da7bdf8c
diff --git a/src/stack.cc b/src/stack.cc
index c32b822..8672975 100644
--- a/src/stack.cc
+++ b/src/stack.cc
@@ -47,7 +47,7 @@
   if (m->IsStatic()) {
     return NULL;
   } else {
-    return GetVRegReference(number_of_vregs_ - num_ins);
+    return GetVRegReference(NumberOfVRegs() - num_ins);
   }
 }