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_stream.cc b/quic/core/quic_crypto_stream.cc
index aef431d..6a941e7 100644
--- a/quic/core/quic_crypto_stream.cc
+++ b/quic/core/quic_crypto_stream.cc
@@ -107,7 +107,7 @@
 bool QuicCryptoStream::ExportKeyingMaterial(QuicStringPiece label,
                                             QuicStringPiece context,
                                             size_t result_len,
-                                            QuicString* result) const {
+                                            std::string* result) const {
   if (!handshake_confirmed()) {
     QUIC_DLOG(ERROR) << "ExportKeyingMaterial was called before forward-secure"
                      << "encryption was established.";