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_session.cc b/quic/core/quic_session.cc
index 09e3865..151fa18 100644
--- a/quic/core/quic_session.cc
+++ b/quic/core/quic_session.cc
@@ -289,7 +289,7 @@
 }
 
 void QuicSession::OnConnectionClosed(QuicErrorCode error,
-                                     const QuicString& error_details,
+                                     const std::string& error_details,
                                      ConnectionCloseSource source) {
   DCHECK(!connection_->connected());
   if (error_ == QUIC_NO_ERROR) {
@@ -617,7 +617,7 @@
 }
 
 void QuicSession::SendGoAway(QuicErrorCode error_code,
-                             const QuicString& reason) {
+                             const std::string& reason) {
   // GOAWAY frame is not supported in v99.
   DCHECK_NE(QUIC_VERSION_99, connection_->transport_version());
   if (goaway_sent_) {