[Identity] Use const ref on ExtensionTokenKey parameter

chrome/browser/extensions/api/identity/extension_token_key.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/extensions/api/identity/extension_token_key.h"
 
 namespace extensions {
 
 ExtensionTokenKey::ExtensionTokenKey(const std::string& extension_id,
                                      const std::string& account_id,
-                                     const std::set<std::string> scopes)
+                                     const std::set<std::string>& scopes)
     : extension_id(extension_id), account_id(account_id), scopes(scopes) {}
 
 ExtensionTokenKey::~ExtensionTokenKey() {}
 
 bool ExtensionTokenKey::operator<(const ExtensionTokenKey& rhs) const {
   if (extension_id < rhs.extension_id)
     return true;
   else if (rhs.extension_id < extension_id)
     return false;
 
   if (account_id < rhs.account_id)
     return true;
   else if (rhs.account_id < account_id)
     return false;
 
   return scopes < rhs.scopes;
 }
 
 }  // namespace extensions