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/test_tools/packet_dropping_test_writer.h b/quic/test_tools/packet_dropping_test_writer.h
index 9537944..2843979 100644
--- a/quic/test_tools/packet_dropping_test_writer.h
+++ b/quic/test_tools/packet_dropping_test_writer.h
@@ -141,7 +141,7 @@
     DelayedWrite& operator=(DelayedWrite&&) = default;
     ~DelayedWrite();
 
-    QuicString buffer;
+    std::string buffer;
     QuicIpAddress self_address;
     QuicSocketAddress peer_address;
     std::unique_ptr<PerPacketOptions> options;