Use std::string instead of string in a few places in QUIC to permit chrome merge.

gfe-relnote: n/a - just adding std:: prefix.
PiperOrigin-RevId: 239062252
Change-Id: I7351ad10d7445eb2c9dd1fc7a6bb5e91c09273a9
diff --git a/quic/core/quic_connection.cc b/quic/core/quic_connection.cc
index ffc4093..3ead1d2 100644
--- a/quic/core/quic_connection.cc
+++ b/quic/core/quic_connection.cc
@@ -1998,7 +1998,7 @@
     // Configured to accept any packet number in range 1...0x7fffffff as initial
     // packet number.
     bool out_of_bound = false;
-    string error_detail = "Packet number out of bounds.";
+    std::string error_detail = "Packet number out of bounds.";
     if (last_header_.packet_number.IsInitialized()) {
       out_of_bound = !Near(packet_number, last_header_.packet_number);
     } else if ((packet_number > MaxRandomInitialPacketNumber())) {