Handle ExternalStrings directly in the serializer without ObjectVisitor.

src/heap/mark-compact.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/heap/mark-compact.h"
 
 #include "src/base/atomicops.h"
 #include "src/base/bits.h"
 #include "src/base/sys-info.h"
 #include "src/code-stubs.h"
@@ skipping 1660 matching lines @@
     DCHECK(RelocInfo::IsCodeAgeSequence(rinfo->rmode()));
     Code* stub = rinfo->code_age_stub();
     USE(stub);
     DCHECK(!Page::FromAddress(stub->address())->IsEvacuationCandidate());
   }
 
   // Entries that are skipped for recording.
   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 {}
 
  private:
   inline void RecordMigratedSlot(Object* value, Address slot) {
     if (value->IsHeapObject()) {
       Page* p = Page::FromAddress(reinterpret_cast<Address>(value));
       if (p->InNewSpace()) {
         RememberedSet<OLD_TO_NEW>::Insert(Page::FromAddress(slot), slot);
       } else if (p->IsEvacuationCandidate()) {
@@ skipping 2474 matching lines @@
       // The target is always in old space, we don't have to record the slot in
       // the old-to-new remembered set.
       DCHECK(!heap()->InNewSpace(target));
       RecordRelocSlot(host, &rinfo, target);
     }
   }
 }
 
 }  // namespace internal
 }  // namespace v8