Add infrastructure to keep track of references to prototypes.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 2ba1e36ddea5f1182f916e693ecad940a59de505..50fe52dd8d70eea68307361814100fb84ef9506d 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -284,6 +284,7 @@ namespace internal {
   V(frozen_symbol)                  \
   V(nonexistent_symbol)             \
   V(elements_transition_symbol)     \
+  V(prototype_users_symbol)         \
   V(observed_symbol)                \
   V(uninitialized_symbol)           \
   V(megamorphic_symbol)             \