[Identity] Use const ref on ExtensionTokenKey parameter

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

 // 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.
 
 #ifndef CHROME_BROWSER_EXTENSIONS_API_IDENTITY_EXTENSION_TOKEN_KEY_H_
 #define CHROME_BROWSER_EXTENSIONS_API_IDENTITY_EXTENSION_TOKEN_KEY_H_
 
 #include <set>
 #include <string>
 
 namespace extensions {
 
 struct ExtensionTokenKey {
   ExtensionTokenKey(const std::string& extension_id,
                     const std::string& account_id,
-                    const std::set<std::string> scopes);
+                    const std::set<std::string>& scopes);
   ~ExtensionTokenKey();
   bool operator<(const ExtensionTokenKey& rhs) const;
   std::string extension_id;
   std::string account_id;
   std::set<std::string> scopes;
 };
 
 }  // namespace extensions
 
 
 #endif  // CHROME_BROWSER_EXTENSIONS_API_IDENTITY_EXTENSION_TOKEN_KEY_H_