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/mock_quic_client_promised_info.h b/quic/test_tools/mock_quic_client_promised_info.h
index a157ac5..5b718c0 100644
--- a/quic/test_tools/mock_quic_client_promised_info.h
+++ b/quic/test_tools/mock_quic_client_promised_info.h
@@ -19,7 +19,7 @@
  public:
   MockQuicClientPromisedInfo(QuicSpdyClientSessionBase* session,
                              QuicStreamId id,
-                             QuicString url);
+                             std::string url);
   ~MockQuicClientPromisedInfo() override;
 
   MOCK_METHOD2(HandleClientRequest,