cros: Add a comment to SystemTrayDelegateChromeOS

The class is deprecated, so make a note of that.

TBR=stevenjb@chromium.org

Bug: none
Test: none
Change-Id: I77e1125dd0ef8d221638b82b99df5cbf4bbb8654
Reviewed-on: https://chromium-review.googlesource.com/579658Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488280}

chrome/browser/ui/ash/system_tray_delegate_chromeos.h

@@ -31,6 +31,10 @@ class SystemTrayNotifier;
 
 namespace chromeos {
 
+// DEPRECATED. Do not add new code here. This class is being removed as part of
+// the transition to mustash. New code should be added to SystemTrayClient.
+// Use system_tray.mojom methods if you need to send information to ash.
+// Please contact jamescook@chromium.org if you have questions or need help.
 class SystemTrayDelegateChromeOS
     : public ash::SystemTrayDelegate,
       public content::NotificationObserver,