Add countdown classes

osu.Game/Online/Multiplayer/GameplayStartCountdown.cs

@@ -0,0 +1,15 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using MessagePack;
+
+namespace osu.Game.Online.Multiplayer
+{
+    /// <summary>
+    /// A <see cref="MultiplayerCountdown"/> indicating that gameplay will start after a given period of time.
+    /// </summary>
+    [MessagePackObject]
+    public class GameplayStartCountdown : MultiplayerCountdown
+    {
+    }
+}

osu.Game/Online/Multiplayer/MultiplayerCountdown.cs

@@ -14,6 +14,7 @@ namespace osu.Game.Online.Multiplayer
     /// </summary>
     [MessagePackObject]
     [Union(0, typeof(MatchStartCountdown))] // IMPORTANT: Add rules to SignalRUnionWorkaroundResolver for new derived types.
+    [Union(1, typeof(GameplayStartCountdown))]
     public abstract class MultiplayerCountdown
     {
         /// <summary>

osu.Game/Online/SignalRWorkaroundTypes.cs

@@ -24,7 +24,8 @@ namespace osu.Game.Online
             (typeof(CountdownChangedEvent), typeof(MatchServerEvent)),
             (typeof(TeamVersusRoomState), typeof(MatchRoomState)),
             (typeof(TeamVersusUserState), typeof(MatchUserState)),
-            (typeof(MatchStartCountdown), typeof(MultiplayerCountdown))
+            (typeof(MatchStartCountdown), typeof(MultiplayerCountdown)),
+            (typeof(GameplayStartCountdown), typeof(MultiplayerCountdown))
         };
     }
 }