Handle ExternalStrings directly in the serializer without ObjectVisitor.

src/snapshot/serializer.h

Index: src/snapshot/serializer.h
diff --git a/src/snapshot/serializer.h b/src/snapshot/serializer.h
index f00b1e1f060ee412965bf682b21d6d6575c39bab..a5131acbf9f9419e2afffdebe34c9d72f169e133 100644
--- a/src/snapshot/serializer.h
+++ b/src/snapshot/serializer.h
@@ -290,24 +290,12 @@ class Serializer::ObjectSerializer : public ObjectVisitor {
   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
@@ -315,8 +303,12 @@ class Serializer::ObjectSerializer : public ObjectVisitor {
   // 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();