Handle ExternalStrings directly in the serializer without ObjectVisitor.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 646cba771ba94a46e8851ddac2e6c0663145c7b2..211f0a7adc6d913df525ced95dd3b6d17e02fe0c 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -10198,12 +10198,6 @@ class ObjectVisitor BASE_EMBEDDED {
   // Visits a runtime entry in the instruction stream.
   virtual void VisitRuntimeEntry(RelocInfo* rinfo) {}
 
-  // Visits the resource of an one-byte or two-byte string.
-  virtual void VisitExternalOneByteString(
-      v8::String::ExternalOneByteStringResource** resource) {}
-  virtual void VisitExternalTwoByteString(
-      v8::String::ExternalStringResource** resource) {}
-
   // Visits a debug call target in the instruction stream.
   virtual void VisitDebugTarget(RelocInfo* rinfo);