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.h b/quic/tools/quic_client_epoll_network_helper.h
index e0f8cf3..d66dbb1 100644
--- a/quic/tools/quic_client_epoll_network_helper.h
+++ b/quic/tools/quic_client_epoll_network_helper.h
@@ -44,7 +44,7 @@
   ~QuicClientEpollNetworkHelper() override;
 
   // Return a name describing the class for use in debug/error reporting.
-  QuicString Name() const override;
+  std::string Name() const override;
 
   // From EpollCallbackInterface
   void OnRegistration(QuicEpollServer* eps, int fd, int event_mask) override;