Moves QuicTag typedefs, and QuicTag utility methods to quic_tag.{h,cc}. No behavior change.

net/quic/test_tools/crypto_test_utils.cc

Index: net/quic/test_tools/crypto_test_utils.cc
diff --git a/net/quic/test_tools/crypto_test_utils.cc b/net/quic/test_tools/crypto_test_utils.cc
index 38f07ed95f8d2373a2a54aa3f5030ce9de736f05..132c0550cb08a647228b7cf433081df9ed3a0dbb 100644
--- a/net/quic/test_tools/crypto_test_utils.cc
+++ b/net/quic/test_tools/crypto_test_utils.cc
@@ -959,7 +959,7 @@ CryptoHandshakeMessage CryptoTestUtils::GenerateDefaultInchoateCHLO(
       "PUBS", CryptoTestUtils::GenerateClientPublicValuesHex().c_str(),
       "NONC", CryptoTestUtils::GenerateClientNonceHex(clock,
                                                       crypto_config).c_str(),
-      "VER\0", QuicUtils::TagToString(
+      "VER\0", QuicTagToString(
           QuicVersionToQuicTag(version)).c_str(),
       "$padding", static_cast<int>(kClientHelloMinimumSize),
       nullptr);