Mark ArcNotificationUserActionData.top_level_notification_key deprecated.

It's not used anywhere in Chromium.

Bug: b:161129562
Change-Id: I14c8414d58c6e1bd2dec3cd666a99418bdf2a6a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2310269Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Yuichiro Hanada <yhanada@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790729}

components/arc/mojom/notifications.mojom

@@ -176,9 +176,9 @@ struct ArcDoNotDisturbStatus {
 struct ArcNotificationUserActionData {
   // Unique action id for this user action.
   uint32 action_id;
-  // Notification key of top-level notification which contains the notification
-  // which is initiating this action.
-  string top_level_notification_key;
+  // DEPRECATED: Notification key of top-level notification which contains
+  // the notification which is initiating this action.
+  string top_level_notification_key_deprecated;
   // True if the task should be executed on the unlocked state.
   bool defer_until_unlock;
   // True if the notification view should be focused after unlock. This is