Make JSONArray::at() a const member function.

third_party/WebKit/Source/platform/json/JSONValues.cpp

 /*
  * Copyright (C) 2010 Google Inc. 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
@@ skipping 465 matching lines @@
 void JSONArray::pushObject(std::unique_ptr<JSONObject> value) {
   DCHECK(value);
   m_data.push_back(std::move(value));
 }
 
 void JSONArray::pushArray(std::unique_ptr<JSONArray> value) {
   DCHECK(value);
   m_data.push_back(std::move(value));
 }
 
-JSONValue* JSONArray::at(size_t index) {
+JSONValue* JSONArray::at(size_t index) const {
   DCHECK_LT(index, m_data.size());
   return m_data[index].get();
 }
 
 }  // namespace blink