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_stream.h b/quic/core/quic_stream.h
index fe0a794..21d57b5 100644
--- a/quic/core/quic_stream.h
+++ b/quic/core/quic_stream.h
@@ -59,7 +59,7 @@
   void AddBytesConsumed(QuicByteCount bytes) override;
   void Reset(QuicRstStreamErrorCode error) override;
   void CloseConnectionWithDetails(QuicErrorCode error,
-                                  const QuicString& details) override;
+                                  const std::string& details) override;
   QuicStreamId id() const override;
   const QuicSocketAddress& PeerAddressOfLatestPacket() const override;
 
@@ -144,7 +144,7 @@
   // Called by the subclass or the sequencer to close the entire connection from
   // this end.
   void CloseConnectionWithDetails(QuicErrorCode error,
-                                  const QuicString& details) override;
+                                  const std::string& details) override;
 
   // Called by the stream sequencer as bytes are consumed from the buffer.
   // If the receive window has dropped below the threshold, then send a