Remove some duplicate code in quic tests.

net/quic/test_tools/quic_test_utils.cc

Index: net/quic/test_tools/quic_test_utils.cc
diff --git a/net/quic/test_tools/quic_test_utils.cc b/net/quic/test_tools/quic_test_utils.cc
index dd415264d6e4fa704b5f0a7b27814c552cb0c7f6..79781a7aebe293b3bc1ccf3d7100c88af2c87f39 100644
--- a/net/quic/test_tools/quic_test_utils.cc
+++ b/net/quic/test_tools/quic_test_utils.cc
@@ -5,6 +5,7 @@
 #include "net/quic/test_tools/quic_test_utils.h"
 
 #include "base/stl_util.h"
+#include "base/strings/string_number_conversions.h"
 #include "net/quic/crypto/crypto_framer.h"
 #include "net/quic/crypto/crypto_handshake.h"
 #include "net/quic/crypto/crypto_utils.h"
@@ -386,6 +387,18 @@ void CompareCharArraysWithHexError(
       << HexDumpWithMarks(actual, actual_len, marks.get(), max_len);
 }
 
+bool DecodeHexString(const base::StringPiece& hex, std::string* bytes) {
+  bytes->clear();
+  if (hex.empty())
+    return true;
+  std::vector<uint8> v;
+  if (!base::HexStringToBytes(hex.as_string(), &v))
+    return false;
+  if (!v.empty())
+    bytes->assign(reinterpret_cast<const char*>(&v[0]), v.size());
+  return true;
+}
+
 static QuicPacket* ConstructPacketFromHandshakeMessage(
     QuicGuid guid,
     const CryptoHandshakeMessage& message,