Adds CryptoHandshakeMessage::HasStringPiece method.

net/quic/core/crypto/crypto_handshake_message.cc

Index: net/quic/core/crypto/crypto_handshake_message.cc
diff --git a/net/quic/core/crypto/crypto_handshake_message.cc b/net/quic/core/crypto/crypto_handshake_message.cc
index 6d09164549a4118010fedcd3f65a8a4d7e0eb3b6..9fcf6ff0d640cd1f11aa42bd875d7cc829844fa4 100644
--- a/net/quic/core/crypto/crypto_handshake_message.cc
+++ b/net/quic/core/crypto/crypto_handshake_message.cc
@@ -6,6 +6,7 @@
 
 #include <memory>
 
+#include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/stringprintf.h"
 #include "net/quic/core/crypto/crypto_framer.h"
@@ -110,6 +111,10 @@ bool CryptoHandshakeMessage::GetStringPiece(QuicTag tag,
   return true;
 }
 
+bool CryptoHandshakeMessage::HasStringPiece(QuicTag tag) const {
+  return base::ContainsKey(tag_value_map_, tag);
+}
+
 QuicErrorCode CryptoHandshakeMessage::GetNthValue24(QuicTag tag,
                                                     unsigned index,
                                                     StringPiece* out) const {