NativeContext::map_cache reference should be strong in heap snapshots

src/heap-snapshot-generator.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1260 matching lines @@
       VariableMode mode;
       int idx = scope_info->FunctionContextSlotIndex(name, &mode);
       if (idx >= 0) {
         SetContextReference(context, entry, name, context->get(idx),
                             Context::OffsetOfElementAt(idx));
       }
     }
   }
 
 #define EXTRACT_CONTEXT_FIELD(index, type, name) \
-  if (Context::index < Context::FIRST_WEAK_SLOT) { \
+  if (Context::index < Context::FIRST_WEAK_SLOT || \
+      Context::index == Context::MAP_CACHE_INDEX) { \
     SetInternalReference(context, entry, #name, context->get(Context::index), \
         FixedArray::OffsetOfElementAt(Context::index)); \
   } else { \
     SetWeakReference(context, entry, #name, context->get(Context::index), \
         FixedArray::OffsetOfElementAt(Context::index)); \
   }
   EXTRACT_CONTEXT_FIELD(CLOSURE_INDEX, JSFunction, closure);
   EXTRACT_CONTEXT_FIELD(PREVIOUS_INDEX, Context, previous);
   EXTRACT_CONTEXT_FIELD(EXTENSION_INDEX, Object, extension);
   EXTRACT_CONTEXT_FIELD(GLOBAL_OBJECT_INDEX, GlobalObject, global);
@@ skipping 1783 matching lines @@
   writer_->AddString("\"<dummy>\"");
   for (int i = 1; i < sorted_strings.length(); ++i) {
     writer_->AddCharacter(',');
     SerializeString(sorted_strings[i]);
     if (writer_->aborted()) return;
   }
 }
 
 
 } }  // namespace v8::internal