Handle ExternalStrings directly in the serializer without ObjectVisitor.

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index 19b2eb66992874cfa24e6a5fb9e545d1a68178f6..2318c5c484db53140ff223d12388080a91c6f0dc 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -1678,10 +1678,6 @@ class RecordMigratedSlotVisitor final : public ObjectVisitor {
   inline void VisitExternalReference(RelocInfo* rinfo) final {}
   inline void VisitExternalReference(Address* p) final {}
   inline void VisitRuntimeEntry(RelocInfo* rinfo) final {}
-  inline void VisitExternalOneByteString(
-      v8::String::ExternalOneByteStringResource** resource) final {}
-  inline void VisitExternalTwoByteString(
-      v8::String::ExternalStringResource** resource) final {}
   inline void VisitInternalReference(RelocInfo* rinfo) final {}
   inline void VisitEmbedderReference(Object** p, uint16_t class_id) final {}