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_client_handshaker.h b/quic/core/quic_crypto_client_handshaker.h
index 5836ea2..d335628 100644
--- a/quic/core/quic_crypto_client_handshaker.h
+++ b/quic/core/quic_crypto_client_handshaker.h
@@ -40,7 +40,7 @@
   int num_scup_messages_received() const override;
   bool WasChannelIDSent() const override;
   bool WasChannelIDSourceCallbackRun() const override;
-  QuicString chlo_hash() const override;
+  std::string chlo_hash() const override;
   bool encryption_established() const override;
   bool handshake_confirmed() const override;
   const QuicCryptoNegotiatedParameters& crypto_negotiated_params()
@@ -87,7 +87,7 @@
 
     // ProofVerifierCallback interface.
     void Run(bool ok,
-             const QuicString& error_details,
+             const std::string& error_details,
              std::unique_ptr<ProofVerifyDetails>* details) override;
 
     // Cancel causes any future callbacks to be ignored. It must be called on
@@ -174,7 +174,7 @@
   QuicCryptoClientConfig* const crypto_config_;
 
   // SHA-256 hash of the most recently sent CHLO.
-  QuicString chlo_hash_;
+  std::string chlo_hash_;
 
   // Server's (hostname, port, is_https, privacy_mode) tuple.
   const QuicServerId server_id_;
@@ -213,7 +213,7 @@
   // verification. These members must not be used after
   // STATE_VERIFY_PROOF_COMPLETE.
   bool verify_ok_;
-  QuicString verify_error_details_;
+  std::string verify_error_details_;
   std::unique_ptr<ProofVerifyDetails> verify_details_;
 
   // True if the server responded to a previous CHLO with a stateless