Fix all Performance and Bug-Prone patterns in quic/core. No functional change. Not flag protected.

net/quic/core/quic_crypto_client_stream_test.cc

 // Copyright (c) 2012 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 "net/quic/core/quic_crypto_client_stream.h"
 
 #include <memory>
 
 #include "net/quic/core/crypto/aes_128_gcm_12_encrypter.h"
 #include "net/quic/core/crypto/quic_decrypter.h"
@@ skipping 207 matching lines @@
   server_config_update.SetValue(kSTTL, expiry_seconds);
 
   std::unique_ptr<QuicData> data(
       CryptoFramer::ConstructHandshakeMessage(server_config_update));
   stream()->OnStreamFrame(QuicStreamFrame(kCryptoStreamId, /*fin=*/false,
                                           /*offset=*/0, data->AsStringPiece()));
 
   // Make sure that the STK and SCFG are cached correctly.
   EXPECT_EQ("xstk", state->source_address_token());
 
-  string cached_scfg = state->server_config();
+  const string& cached_scfg = state->server_config();
   test::CompareCharArraysWithHexError(
       "scfg", cached_scfg.data(), cached_scfg.length(),
       QuicUtils::AsChars(scfg), arraysize(scfg));
 }
 
 TEST_F(QuicCryptoClientStreamTest, ServerConfigUpdateBeforeHandshake) {
   EXPECT_CALL(
       *connection_,
       CloseConnection(QUIC_CRYPTO_UPDATE_BEFORE_HANDSHAKE_COMPLETE, _, _));
   CryptoHandshakeMessage server_config_update;
@@ skipping 135 matching lines @@
       client_state->GetNextServerDesignatedConnectionId();
   QuicConnectionId expected_id =
       server_session_->connection()->random_generator()->RandUint64();
   EXPECT_EQ(expected_id, server_designated_id);
   EXPECT_FALSE(client_state->has_server_designated_connection_id());
 }
 
 }  // namespace
 }  // namespace test
 }  // namespace net