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

net/quic/crypto/crypto_server_config_protobuf.h

Index: net/quic/crypto/crypto_server_config_protobuf.h
diff --git a/net/quic/crypto/crypto_server_config_protobuf.h b/net/quic/crypto/crypto_server_config_protobuf.h
index 57ebfb048478efc4b2bf2ab63ae30003c2a2f767..fca5f0a51b8bf26658821a4ad8d26a434acca00a 100644
--- a/net/quic/crypto/crypto_server_config_protobuf.h
+++ b/net/quic/crypto/crypto_server_config_protobuf.h
@@ -35,9 +35,7 @@ class NET_EXPORT_PRIVATE QuicServerConfigProtobuf {
     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_;