Extract non-IO part of mksnapshot into an API method.

src/factory.cc

Index: src/factory.cc
diff --git a/src/factory.cc b/src/factory.cc
index 4680e75204b30bf91770ab04333fa9fcdd7b94d0..7e6a1d87b6290bc15ac16ac49ba20f7d5b1153e4 100644
--- a/src/factory.cc
+++ b/src/factory.cc
@@ -2156,7 +2156,7 @@ void Factory::SetNumberStringCache(Handle<Object> number,
       // cache in the snapshot to keep  boot-time memory usage down.
       // If we expand the number string cache already while creating
       // the snapshot then that didn't work out.
-      DCHECK(!isolate()->serializer_enabled() || FLAG_extra_code != NULL);
+      DCHECK(!isolate()->serializer_enabled());
       Handle<FixedArray> new_cache = NewFixedArray(full_size, TENURED);
       isolate()->heap()->set_number_string_cache(*new_cache);
       return;