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

Index: net/quic/core/quic_crypto_client_stream_test.cc
diff --git a/net/quic/core/quic_crypto_client_stream_test.cc b/net/quic/core/quic_crypto_client_stream_test.cc
index e877cd5e6f0701c2ec669db186ae18f6b5f0e801..07db441fa5711f5619c58db600fb2d0c63f2ad1e 100644
--- a/net/quic/core/quic_crypto_client_stream_test.cc
+++ b/net/quic/core/quic_crypto_client_stream_test.cc
@@ -225,7 +225,7 @@ TEST_F(QuicCryptoClientStreamTest, ServerConfigUpdate) {
   // 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));