Serialize all external strings except for native source code strings.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index b281cb45451c61db3a8b91f1a4b4516ef09941eb..c37464c9bcd6e9cc5c2fbf34347cc68b2cff4398 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -83,6 +83,7 @@ namespace internal {
   V(Map, external_string_with_one_byte_data_map,                               \
     ExternalStringWithOneByteDataMap)                                          \
   V(Map, external_one_byte_string_map, ExternalOneByteStringMap)               \
+  V(Map, native_source_string_map, NativeSourceStringMap)                      \
   V(Map, short_external_string_map, ShortExternalStringMap)                    \
   V(Map, short_external_string_with_one_byte_data_map,                         \
     ShortExternalStringWithOneByteDataMap)                                     \