Verify a bit more about external strings

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index 56fe98d40415583a335ae62e893e05f7766cdf0d..0c193b003849bc4bb34d1b06c228d644c840e733 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -2428,9 +2428,9 @@ class V8_EXPORT String : public Name {
   };
 
  private:
-  void VerifyExternalStringResourceBase(ExternalStringResourceBase* v,
+  friend class internal::Heap;
+  void VerifyExternalStringResourceBase(const ExternalStringResourceBase* v,
                                         Encoding encoding) const;
-  void VerifyExternalStringResource(ExternalStringResource* val) const;
   static void CheckCast(v8::Value* obj);
 };
 
@@ -7631,7 +7631,7 @@ String::ExternalStringResource* String::GetExternalStringResource() const {
     result = NULL;
   }
 #ifdef V8_ENABLE_CHECKS
-  VerifyExternalStringResource(result);
+  VerifyExternalStringResourceBase(result, TWO_BYTE_ENCODING);
 #endif
   return result;
 }