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/quartc/quartc_endpoint.h b/quic/quartc/quartc_endpoint.h
index ee856ea..abf056a 100644
--- a/quic/quartc/quartc_endpoint.h
+++ b/quic/quartc/quartc_endpoint.h
@@ -43,7 +43,7 @@
     // Called if the endpoint fails to establish a session after a call to
     // Connect.  (The most likely cause is a network idle timeout.)
     virtual void OnConnectError(QuicErrorCode error,
-                                const QuicString& error_details) = 0;
+                                const std::string& error_details) = 0;
   };
 
   virtual ~QuartcEndpoint() = default;
@@ -97,7 +97,7 @@
   QuartcEndpoint::Delegate* delegate_;
 
   // Server config.  If valid, used to perform a 0-RTT connection.
-  const QuicString serialized_server_config_;
+  const std::string serialized_server_config_;
 
   // Version manager.  May be injected to control version negotiation in tests.
   std::unique_ptr<QuicVersionManager> version_manager_;