Add infrastructure to keep track of references to prototypes.

src/heap/heap.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HEAP_HEAP_H_
 #define V8_HEAP_HEAP_H_
 
 #include <cmath>
 
 #include "src/allocation.h"
@@ skipping 266 matching lines @@
   V(Array_string, "Array")                                 \
   V(Error_string, "Error")                                 \
   V(RegExp_string, "RegExp")
 
 #define PRIVATE_SYMBOL_LIST(V)      \
   V(nonextensible_symbol)           \
   V(sealed_symbol)                  \
   V(frozen_symbol)                  \
   V(nonexistent_symbol)             \
   V(elements_transition_symbol)     \
+  V(prototype_users_symbol)         \
   V(observed_symbol)                \
   V(uninitialized_symbol)           \
   V(megamorphic_symbol)             \
   V(premonomorphic_symbol)          \
   V(generic_symbol)                 \
   V(stack_trace_symbol)             \
   V(detailed_stack_trace_symbol)    \
   V(normal_ic_symbol)               \
   V(home_object_symbol)             \
   V(intl_initialized_marker_symbol) \
@@ skipping 2290 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 }
 }  // namespace v8::internal
 
 #endif  // V8_HEAP_HEAP_H_