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/crypto/quic_decrypter.cc b/quic/core/crypto/quic_decrypter.cc
index d2aa28d..1ab67ba 100644
--- a/quic/core/crypto/quic_decrypter.cc
+++ b/quic/core/crypto/quic_decrypter.cc
@@ -55,14 +55,14 @@
                                             const DiversificationNonce& nonce,
                                             size_t key_size,
                                             size_t nonce_prefix_size,
-                                            QuicString* out_key,
-                                            QuicString* out_nonce_prefix) {
-  QuicHKDF hkdf((QuicString(preliminary_key)) + (QuicString(nonce_prefix)),
+                                            std::string* out_key,
+                                            std::string* out_nonce_prefix) {
+  QuicHKDF hkdf((std::string(preliminary_key)) + (std::string(nonce_prefix)),
                 QuicStringPiece(nonce.data(), nonce.size()),
                 "QUIC key diversification", 0, key_size, 0, nonce_prefix_size,
                 0);
-  *out_key = QuicString(hkdf.server_write_key());
-  *out_nonce_prefix = QuicString(hkdf.server_write_iv());
+  *out_key = std::string(hkdf.server_write_key());
+  *out_nonce_prefix = std::string(hkdf.server_write_iv());
 }
 
 }  // namespace quic