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/platform/api/quic_socket_address.cc b/quic/platform/api/quic_socket_address.cc
index fd428e5..982ebf9 100644
--- a/quic/platform/api/quic_socket_address.cc
+++ b/quic/platform/api/quic_socket_address.cc
@@ -31,7 +31,7 @@
   return impl_.IsInitialized();
 }
 
-QuicString QuicSocketAddress::ToString() const {
+std::string QuicSocketAddress::ToString() const {
   return impl_.ToString();
 }