Refactor common QuicTransport constants into their own file.

Also replace them with constexpr functions so that they can be inlined.

gfe-relnote: n/a (no functional change)
PiperOrigin-RevId: 273604469
Change-Id: Ic1f9d1b020afb2505789c7d0b3f93a154df95091

quic/quic_transport/quic_transport_client_session.h

diff --git a/quic/quic_transport/quic_transport_client_session.h b/quic/quic_transport/quic_transport_client_session.h
index 9ae8500..69e7d8e 100644
--- a/quic/quic_transport/quic_transport_client_session.h
+++ b/quic/quic_transport/quic_transport_client_session.h
@@ -20,18 +20,10 @@
 #include "net/third_party/quiche/src/quic/core/quic_versions.h"
 #include "net/third_party/quiche/src/quic/platform/api/quic_bug_tracker.h"
 #include "net/third_party/quiche/src/quic/platform/api/quic_string_piece.h"
+#include "net/third_party/quiche/src/quic/quic_transport/quic_transport_protocol.h"
 
 namespace quic {
 
-// The ALPN used by QuicTransport.
-QUIC_EXPORT extern const char* kQuicTransportAlpn;
-
-QUIC_EXPORT extern const QuicStreamId kClientIndicationStream;
-
-enum class QuicTransportClientIndicationKeys : uint16_t {
-  kOrigin = 0x0000,
-};
-
 // A client session for the QuicTransport protocol.
 class QUIC_EXPORT QuicTransportClientSession : public QuicSession {
  public:
@@ -44,7 +36,7 @@
                              url::Origin origin);
 
   std::vector<std::string> GetAlpnsToOffer() const override {
-    return std::vector<std::string>({kQuicTransportAlpn});
+    return std::vector<std::string>({QuicTransportAlpn()});
   }
 
   void CryptoConnect() { crypto_stream_->CryptoConnect(); }