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_packet_creator.h b/quic/core/quic_packet_creator.h
index f4512c3..3ff5819 100644
--- a/quic/core/quic_packet_creator.h
+++ b/quic/core/quic_packet_creator.h
@@ -390,7 +390,7 @@
   SerializedPacket packet_;
 
   // Retry token to send over the wire in v99 IETF Initial packets.
-  QuicString retry_token_;
+  std::string retry_token_;
 
   // Pending padding bytes to send. Pending padding bytes will be sent in next
   // packet(s) (after all other frames) if current constructed packet does not