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_packets.h b/quic/core/quic_packets.h
index d2aaf34..509fd08 100644
--- a/quic/core/quic_packets.h
+++ b/quic/core/quic_packets.h
@@ -140,7 +140,7 @@
   // An arbitrary string to identify an endpoint. Used by clients to
   // differentiate traffic from Google servers vs Non-google servers.
   // Will not be used if empty().
-  QuicString endpoint_id;
+  std::string endpoint_id;
 };
 
 struct QUIC_EXPORT_PRIVATE QuicVersionNegotiationPacket {