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

net/tools/quic/stateless_rejector_test.cc

Index: net/tools/quic/stateless_rejector_test.cc
diff --git a/net/tools/quic/stateless_rejector_test.cc b/net/tools/quic/stateless_rejector_test.cc
index 3388742d9b291d366fef74adcd1a4971bd19dadf..1d35fba55a51b3309554224f862dbb49da3de7d0 100644
--- a/net/tools/quic/stateless_rejector_test.cc
+++ b/net/tools/quic/stateless_rejector_test.cc
@@ -103,7 +103,7 @@ class StatelessRejectorTest : public ::testing::TestWithParam<TestParams> {
     scid_hex_ = "#" + QuicUtils::HexEncode(config_peer_.GetPrimaryConfig()->id);
 
     // Encode the QUIC version.
-    ver_hex_ = QuicUtils::TagToString(QuicVersionToQuicTag(GetParam().version));
+    ver_hex_ = QuicTagToString(QuicVersionToQuicTag(GetParam().version));
 
     // Generate a public value.
     char public_value[32];