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.h b/quic/core/crypto/crypto_utils.h
index a958311..b881778 100644
--- a/quic/core/crypto/crypto_utils.h
+++ b/quic/core/crypto/crypto_utils.h
@@ -198,8 +198,8 @@
       HandshakeFailureReason reason);
 
   // Returns a hash of the serialized |message|.
-  static string HashHandshakeMessage(const CryptoHandshakeMessage& message,
-                                     Perspective perspective);
+  static std::string HashHandshakeMessage(const CryptoHandshakeMessage& message,
+                                          Perspective perspective);
 
  private:
   // Implements the HKDF-Expand-Label function as defined in section 7.1 of RFC