Cleanup: Pass std::string as const reference if possible

net/quic/crypto/crypto_server_config_protobuf.h

 // Copyright (c) 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 NET_QUIC_CRYPTO_CRYPTO_SERVER_CONFIG_PROTOBUF_H_
 #define NET_QUIC_CRYPTO_CRYPTO_SERVER_CONFIG_PROTOBUF_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 17 matching lines @@
    public:
     QuicTag tag() const {
       return tag_;
     }
     void set_tag(QuicTag tag) {
       tag_ = tag;
     }
     std::string private_key() const {
       return private_key_;
     }
-    void set_private_key(std::string key) {
-      private_key_ = key;
-    }
+    void set_private_key(const std::string& key) { private_key_ = key; }
 
    private:
     QuicTag tag_;
     std::string private_key_;
   };
 
   QuicServerConfigProtobuf();
   ~QuicServerConfigProtobuf();
 
   size_t key_size() const {
@@ skipping 79 matching lines @@
   // tokens when talking to clients that select this server config.
   // It can be of any length as it is fed into a KDF before use.
   std::string source_address_token_secret_override_;
 
   DISALLOW_COPY_AND_ASSIGN(QuicServerConfigProtobuf);
 };
 
 }  // namespace net
 
 #endif  // NET_QUIC_CRYPTO_CRYPTO_SERVER_CONFIG_PROTOBUF_H_