NativeContext::map_cache reference should be strong in heap snapshots

This change makes the reference itself a strong one. We will also need a special handling to make references from MapCache object to Map weak in the heap snapshots. The latter will be addressed separately.

BUG=chromium:357060
LOG=Y
R=alph@chromium.org, hpayer@chromium.org

Review URL: https://codereview.chromium.org/214683002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20304 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

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 { \