NativeContext::map_cache reference should be strong in heap snapshots

src/heap-snapshot-generator.cc

Index: src/heap-snapshot-generator.cc
diff --git a/src/heap-snapshot-generator.cc b/src/heap-snapshot-generator.cc
index 10e177d4f0ecf8425a249d91b2b528af285d0089..332d0dbf6ff3705318f28a666d7134667e12295c 100644
--- a/src/heap-snapshot-generator.cc
+++ b/src/heap-snapshot-generator.cc
@@ -1278,7 +1278,8 @@ void V8HeapExplorer::ExtractContextReferences(int entry, Context* context) {
   }
 
 #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 { \