Return MaybeHandle from NewExternalStringFrom*.

src/handles.h

 // Copyright 2011 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 64 matching lines @@
   INLINE(bool ToHandle(Handle<T>* out)) {
     if (location_ == NULL) {
       *out = Handle<T>::null();
       return false;
     } else {
       *out = Handle<T>(location_);
       return true;
     }
   }
 
+  bool is_null() const { return location_ == NULL; }
+
  protected:
   T** location_;
 
   // MaybeHandles of different classes are allowed to access each
   // other's location_.
   template<class S> friend class MaybeHandle;
 };
 
 // ----------------------------------------------------------------------------
 // A Handle provides a reference to an object that survives relocation by
@@ skipping 284 matching lines @@
 
   void Initialize() {
     next = limit = NULL;
     level = 0;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_HANDLES_H_