Make a Persistent ctor take const Persistent& instead of Persistent&.

include/v8.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 491 matching lines @@
 
   /**
    * A constructor that creates a new global cell pointing to that. In contrast
    * to the copy constructor, this creates a new persistent handle which needs
    * to be separately disposed.
    */
   template <class S> V8_INLINE(Persistent(Isolate* isolate, Handle<S> that))
       : val_(New(isolate, *that)) { }
 
   template <class S> V8_INLINE(Persistent(Isolate* isolate,
-                                          Persistent<S>& that)) // NOLINT
+                                          const Persistent<S>& that)) // NOLINT
       : val_(New(isolate, *that)) { }
 
 #else
   /**
    * Creates an empty persistent handle that doesn't point to any
    * storage cell.
    */
   V8_INLINE(Persistent()) : Handle<T>() { }
 
   /**
@@ skipping 5849 matching lines @@
 
 
 }  // namespace v8
 
 
 #undef V8EXPORT
 #undef TYPE_CHECK
 
 
 #endif  // V8_H_