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/frames/quic_connection_close_frame.h b/quic/core/frames/quic_connection_close_frame.h
index 2bbbe73..1b909ce 100644
--- a/quic/core/frames/quic_connection_close_frame.h
+++ b/quic/core/frames/quic_connection_close_frame.h
@@ -16,9 +16,9 @@
 
 struct QUIC_EXPORT_PRIVATE QuicConnectionCloseFrame {
   QuicConnectionCloseFrame();
-  QuicConnectionCloseFrame(QuicErrorCode error_code, QuicString error_details);
+  QuicConnectionCloseFrame(QuicErrorCode error_code, std::string error_details);
   QuicConnectionCloseFrame(QuicIetfTransportErrorCodes ietf_error_code,
-                           QuicString error_details,
+                           std::string error_details,
                            uint64_t frame_type);
 
   friend QUIC_EXPORT_PRIVATE std::ostream& operator<<(
@@ -33,7 +33,7 @@
     QuicErrorCode error_code;
     QuicIetfTransportErrorCodes ietf_error_code;
   };
-  QuicString error_details;
+  std::string error_details;
 
   // Contains the type of frame that triggered the connection close. Made a
   // uint64, as opposed to the QuicIetfFrameType, to support possible