Support cancelling toast

components/arc/common/notifications.mojom

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module arc.mojom;
 
 import "bitmap.mojom";
 
 // These values must be matched with the NOTIFICATION_EVENT_* constants in
 // com.android.server.ArcNotificationListenerService.
@@ skipping 57 matching lines @@
   // Image for image notifications.
   [MinVersion=3]
   ArcBitmap? big_picture;
 };
 
 [MinVersion=2]
 struct ArcToastData {
   // Unique identifier
   string id;
   // Toast text.
-  string text;
+  string? text;
   // Toast duration in milliseconds.
   int32 duration;
 };
 
 interface NotificationsHost {
   // Tells the Chrome that a notification is posted (created or updated) on
   // Android.
   // |notification_data| is the data of notification (id, texts, icon and ...).
   OnNotificationPosted@0(ArcNotificationData notification_data);
 
@@ skipping 12 matching lines @@
 
 // TODO(lhchavez): Migrate all request/response messages to Mojo.
 interface NotificationsInstance {
   // Establishes full-duplex communication with the host.
   Init(NotificationsHost host_ptr);
 
   // Sends an event from Chrome notification UI to Android.
   // |event| is a type of occured event.
   SendNotificationEventToAndroid(string key, ArcNotificationEvent event);
 };