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/tools/quic_client_epoll_network_helper.cc b/quic/tools/quic_client_epoll_network_helper.cc
index 846911f..e545ee2 100644
--- a/quic/tools/quic_client_epoll_network_helper.cc
+++ b/quic/tools/quic_client_epoll_network_helper.cc
@@ -55,7 +55,7 @@
   CleanUpAllUDPSockets();
 }
 
-QuicString QuicClientEpollNetworkHelper::Name() const {
+std::string QuicClientEpollNetworkHelper::Name() const {
   return "QuicClientEpollNetworkHelper";
 }