Replace QuicString with std::string, pass 1

This replaces QuicString with std::string in all of the "QUIC proper".  I will
delete QuicString once all code using it is gone.

gfe-relnote: n/a (no functional change)
PiperOrigin-RevId: 237872023
Change-Id: I82de62c9855516b15039734d05155917e68ff4ee
diff --git a/quic/core/quic_crypto_client_stream.cc b/quic/core/quic_crypto_client_stream.cc
index 66bb9e4..52edbc0 100644
--- a/quic/core/quic_crypto_client_stream.cc
+++ b/quic/core/quic_crypto_client_stream.cc
@@ -92,7 +92,7 @@
   return handshaker_->WasChannelIDSourceCallbackRun();
 }
 
-QuicString QuicCryptoClientStream::chlo_hash() const {
+std::string QuicCryptoClientStream::chlo_hash() const {
   return handshaker_->chlo_hash();
 }