factor out path id + packet number packing code from the QUIC aead code

net/quic/quic_utils.cc

Index: net/quic/quic_utils.cc
diff --git a/net/quic/quic_utils.cc b/net/quic/quic_utils.cc
index 2c1c24e68114a3d95de02211034f678bffeca352..2213f0bfa8ffb55e1314a6579aaffb061baf73a8 100644
--- a/net/quic/quic_utils.cc
+++ b/net/quic/quic_utils.cc
@@ -464,4 +464,19 @@ void QuicUtils::ClearSerializedPacket(SerializedPacket* serialized_packet) {
   serialized_packet->packet = nullptr;
 }
 
+// static
+uint64_t QuicUtils::PackPathIdAndPacketNumber(QuicPathId path_id,
+                                              QuicPacketNumber packet_number) {
+  // Setting the nonce below relies on QuicPathId and QuicPacketNumber being
+  // specific sizes.
+  static_assert(sizeof(path_id) == 1, "Size of QuicPathId changed.");
+  static_assert(sizeof(packet_number) == 8,
+                "Size of QuicPacketNumber changed.");
+  // Use path_id and lower 7 bytes of packet_number as lower 8 bytes of nonce.
+  uint64_t path_id_packet_number =
+      (static_cast<uint64_t>(path_id) << 56) | packet_number;
+  DCHECK(path_id != kDefaultPathId || path_id_packet_number == packet_number);
+  return path_id_packet_number;
+}
+
 }  // namespace net