Add pointer cache field to external string for access in generated code.

src/serialize.cc

Index: src/serialize.cc
diff --git a/src/serialize.cc b/src/serialize.cc
index 8c19d99895d51581f9131cf27041b5c1324c4498..e3a84c8c9d3b42e3c18fd87e07a62ed41464af18 100644
--- a/src/serialize.cc
+++ b/src/serialize.cc
@@ -1564,6 +1564,7 @@ void Serializer::ObjectSerializer::VisitExternalAsciiString(
         sink_->Put(kNativesStringResource, "NativesStringResource");
         sink_->PutSection(i, "NativesStringResourceEnd");
         bytes_processed_so_far_ += sizeof(resource);
+        string->clear_data_cache();
         return;
       }
     }