[LookupIterator] mark index_ as const

src/lookup.h

 // Copyright 2014 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.
 
 #ifndef V8_LOOKUP_H_
 #define V8_LOOKUP_H_
 
 #include "src/factory.h"
 #include "src/isolate.h"
 #include "src/objects.h"
@@ skipping 350 matching lines @@
   State state_;
   bool has_property_;
   InterceptorState interceptor_state_;
   PropertyDetails property_details_;
   Isolate* const isolate_;
   Handle<Name> name_;
   Handle<Object> transition_;
   const Handle<Object> receiver_;
   Handle<JSReceiver> holder_;
   const Handle<JSReceiver> initial_holder_;
-  uint32_t index_;
+  const uint32_t index_;
   uint32_t number_;
 };
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_LOOKUP_H_