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 edfc1e62b92d69ac5b47aa8d22018e6f3a927f5f..62cba63077e03bf02f7c02184d1ccc32933c1afd 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -282,6 +282,7 @@ 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)             \