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.cc b/quic/core/frames/quic_connection_close_frame.cc
index 51969c6..55d356b 100644
--- a/quic/core/frames/quic_connection_close_frame.cc
+++ b/quic/core/frames/quic_connection_close_frame.cc
@@ -10,14 +10,14 @@
     : error_code(QUIC_NO_ERROR), frame_type(0) {}
 
 QuicConnectionCloseFrame::QuicConnectionCloseFrame(QuicErrorCode error_code,
-                                                   QuicString error_details)
+                                                   std::string error_details)
     : error_code(error_code),
       error_details(std::move(error_details)),
       frame_type(0) {}
 
 QuicConnectionCloseFrame::QuicConnectionCloseFrame(
     QuicIetfTransportErrorCodes ietf_error_code,
-    QuicString error_details,
+    std::string error_details,
     uint64_t frame_type)
     : ietf_error_code(ietf_error_code),
       error_details(std::move(error_details)),