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_goaway_frame.h b/quic/core/frames/quic_goaway_frame.h
index 085e915..03d6273 100644
--- a/quic/core/frames/quic_goaway_frame.h
+++ b/quic/core/frames/quic_goaway_frame.h
@@ -18,7 +18,7 @@
   QuicGoAwayFrame(QuicControlFrameId control_frame_id,
                   QuicErrorCode error_code,
                   QuicStreamId last_good_stream_id,
-                  const QuicString& reason);
+                  const std::string& reason);
 
   friend QUIC_EXPORT_PRIVATE std::ostream& operator<<(std::ostream& os,
                                                       const QuicGoAwayFrame& g);
@@ -28,7 +28,7 @@
   QuicControlFrameId control_frame_id;
   QuicErrorCode error_code;
   QuicStreamId last_good_stream_id;
-  QuicString reason_phrase;
+  std::string reason_phrase;
 };
 
 }  // namespace quic