Handle ExternalStrings directly in the serializer without ObjectVisitor.

src/snapshot/serializer.h

 // Copyright 2016 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.
 
 #ifndef V8_SNAPSHOT_SERIALIZER_H_
 #define V8_SNAPSHOT_SERIALIZER_H_
 
 #include "src/isolate.h"
 #include "src/log.h"
 #include "src/objects.h"
@@ skipping 272 matching lines @@
   void SerializeDeferred();
   void VisitPointers(Object** start, Object** end) override;
   void VisitEmbeddedPointer(RelocInfo* target) override;
   void VisitExternalReference(Address* p) override;
   void VisitExternalReference(RelocInfo* rinfo) override;
   void VisitInternalReference(RelocInfo* rinfo) override;
   void VisitCodeTarget(RelocInfo* target) override;
   void VisitCodeEntry(Address entry_address) override;
   void VisitCell(RelocInfo* rinfo) override;
   void VisitRuntimeEntry(RelocInfo* reloc) override;
-  // Used for seralizing the external strings that hold the natives source.
-  void VisitExternalOneByteString(
-      v8::String::ExternalOneByteStringResource** resource) override;
-  // We can't serialize a heap with external two byte strings.
-  void VisitExternalTwoByteString(
-      v8::String::ExternalStringResource** resource) override {
-    UNREACHABLE();
-  }
 
  private:
   bool TryEncodeDeoptimizationEntry(HowToCode how_to_code, Address target,
                                     int skip);
   void SerializePrologue(AllocationSpace space, int size, Map* map);
 
-  bool SerializeExternalNativeSourceString(
-      int builtin_count,
-      v8::String::ExternalOneByteStringResource** resource_pointer,
-      FixedArray* source_cache, int resource_index);
 
   enum ReturnSkip { kCanReturnSkipInsteadOfSkipping, kIgnoringReturn };
   // This function outputs or skips the raw data between the last pointer and
   // up to the current position.  It optionally can just return the number of
   // bytes to skip instead of performing a skip instruction, in case the skip
   // can be merged into the next instruction.
   int OutputRawData(Address up_to, ReturnSkip return_skip = kIgnoringReturn);
-  // External strings are serialized in a way to resemble sequential strings.
   void SerializeExternalString();
+  void SerializeExternalStringAsSequentialString();
+  bool SerializeExternalNativeSourceString(
+      int builtin_count,
+      v8::String::ExternalOneByteStringResource** resource_pointer,
+      FixedArray* source_cache, int resource_index);
 
   Address PrepareCode();
 
   Serializer* serializer_;
   HeapObject* object_;
   SnapshotByteSink* sink_;
   int reference_representation_;
   int bytes_processed_so_far_;
   bool code_has_been_output_;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SNAPSHOT_SERIALIZER_H_