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.h b/quic/platform/api/quic_socket_address.h
index 297387f..21f29e4 100644
--- a/quic/platform/api/quic_socket_address.h
+++ b/quic/platform/api/quic_socket_address.h
@@ -31,7 +31,7 @@
                                              const QuicSocketAddress& rhs);
 
   bool IsInitialized() const;
-  QuicString ToString() const;
+  std::string ToString() const;
   int FromSocket(int fd);
   QuicSocketAddress Normalized() const;