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_connection_id.h b/quic/core/quic_connection_id.h
index 5a78acd..5a63088 100644
--- a/quic/core/quic_connection_id.h
+++ b/quic/core/quic_connection_id.h
@@ -61,7 +61,7 @@
 
   // Generates an ASCII string that represents
   // the contents of the connection ID, or "0" if it is empty.
-  QuicString ToString() const;
+  std::string ToString() const;
 
   // operator<< allows easily logging connection IDs.
   friend QUIC_EXPORT_PRIVATE std::ostream& operator<<(