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.cc b/quic/core/quic_utils.cc
index d83db1c..10854bf 100644
--- a/quic/core/quic_utils.cc
+++ b/quic/core/quic_utils.cc
@@ -165,7 +165,7 @@
   return "INVALID_TRANSMISSION_TYPE";
 }
 
-QuicString QuicUtils::AddressChangeTypeToString(AddressChangeType type) {
+std::string QuicUtils::AddressChangeTypeToString(AddressChangeType type) {
   switch (type) {
     RETURN_STRING_LITERAL(NO_CHANGE);
     RETURN_STRING_LITERAL(PORT_CHANGE);