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_server_stream.cc b/quic/core/quic_crypto_server_stream.cc
index aa75c66..02e7b8f 100644
--- a/quic/core/quic_crypto_server_stream.cc
+++ b/quic/core/quic_crypto_server_stream.cc
@@ -71,7 +71,7 @@
 }
 
 bool QuicCryptoServerStream::GetBase64SHA256ClientChannelID(
-    QuicString* output) const {
+    std::string* output) const {
   return handshaker()->GetBase64SHA256ClientChannelID(output);
 }