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_framer.h b/quic/core/quic_framer.h
index 168fef5..d6916f6 100644
--- a/quic/core/quic_framer.h
+++ b/quic/core/quic_framer.h
@@ -516,7 +516,7 @@
   // to ciphertext no larger than |ciphertext_size|.
   size_t GetMaxPlaintextSize(size_t ciphertext_size);
 
-  const QuicString& detailed_error() { return detailed_error_; }
+  const std::string& detailed_error() { return detailed_error_; }
 
   // The minimum packet number length required to represent |packet_number|.
   static QuicPacketNumberLength GetMinPacketNumberLength(
@@ -859,7 +859,7 @@
 
   void set_detailed_error(const char* error) { detailed_error_ = error; }
 
-  QuicString detailed_error_;
+  std::string detailed_error_;
   QuicFramerVisitorInterface* visitor_;
   QuicErrorCode error_;
   // Updated by ProcessPacketHeader when it succeeds decrypting a larger packet.