Fix missing cases of empty_string canonicalization

include/v8.h

 // 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.
 
 /** \mainpage V8 API Reference Guide
  *
  * V8 is Google's open source JavaScript engine.
  *
  * This set of documents provides reference material generated from the
  * V8 header file, include/v8.h.
@@ skipping 2429 matching lines @@
      * control how allocated external string resources are disposed.
      */
     virtual void Dispose() { delete this; }
 
     // Disallow copying and assigning.
     ExternalStringResourceBase(const ExternalStringResourceBase&) = delete;
     void operator=(const ExternalStringResourceBase&) = delete;
 
    private:
     friend class internal::Heap;
+    friend class v8::String;
   };
 
   /**
    * An ExternalStringResource is a wrapper around a two-byte string
    * buffer that resides outside V8's heap. Implement an
    * ExternalStringResource to manage the life cycle of the underlying
    * buffer.  Note that the string data must be immutable.
    */
   class V8_EXPORT ExternalStringResource
       : public ExternalStringResourceBase {
@@ skipping 7404 matching lines @@
  */
 
 
 }  // namespace v8
 
 
 #undef TYPE_CHECK
 
 
 #endif  // INCLUDE_V8_H_