Add chat toggle button.

osu.Game/Overlays/Toolbar/ToolbarChatButton.cs

@@ -0,0 +1,23 @@
+// Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using osu.Framework.Allocation;
+using osu.Game.Graphics;
+
+namespace osu.Game.Overlays.Toolbar
+{
+    internal class ToolbarChatButton : ToolbarOverlayToggleButton
+    {
+        public ToolbarChatButton()
+        {
+            Icon = FontAwesome.fa_comments;
+        }
+
+        [BackgroundDependencyLoader]
+        private void load(ChatOverlay chat)
+        {
+            StateContainer = chat;
+            Action = chat.ToggleVisibility;
+        }
+    }
+}