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/crypto/crypto_utils.cc b/quic/core/crypto/crypto_utils.cc
index e3e438b..7580bbb 100644
--- a/quic/core/crypto/crypto_utils.cc
+++ b/quic/core/crypto/crypto_utils.cc
@@ -458,7 +458,7 @@
 std::string CryptoUtils::HashHandshakeMessage(
     const CryptoHandshakeMessage& message,
     Perspective perspective) {
-  string output;
+  std::string output;
   const QuicData& serialized = message.GetSerialized();
   uint8_t digest[SHA256_DIGEST_LENGTH];
   SHA256(reinterpret_cast<const uint8_t*>(serialized.data()),