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_utils.h b/quic/core/quic_utils.h
index 28ea15b..3ef4788 100644
--- a/quic/core/quic_utils.h
+++ b/quic/core/quic_utils.h
@@ -57,7 +57,7 @@
   static const char* TransmissionTypeToString(TransmissionType type);
 
   // Returns AddressChangeType as a string.
-  static QuicString AddressChangeTypeToString(AddressChangeType type);
+  static std::string AddressChangeTypeToString(AddressChangeType type);
 
   // Returns SentPacketState as a char*.
   static const char* SentPacketStateToString(SentPacketState state);