[Presentation API] Plumbing of |onstatechange| from the Mojo service to the public/platform

content/common/presentation/presentation_service.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 presentation;
 
 struct PresentationSessionInfo {
     string url;
     string id;
 };
 
+enum PresentationSessionState {
+    CONNECTED,
+    DISCONNECTED
+};
+
 enum PresentationErrorType {
     NO_AVAILABLE_SCREENS,
     SESSION_REQUEST_CANCELLED,
     NO_PRESENTATION_FOUND,
     UNKNOWN,
 };
 
 struct PresentationError {
     PresentationErrorType error_type;
     string message;
@@ skipping 43 matching lines @@
 
     // Called when joinSession() is called by the frame. The result callback
     // works the same as for the method above. JoinSession will join a known
     // session (i.e. when the page navigates or the user opens another tab)
     // silently and without user action.
     JoinSession(string presentation_url, string? presentation_id)
         => (PresentationSessionInfo? sessionInfo, PresentationError? error);
 
     // Called when closeSession() is called by the frame.
     CloseSession(string presentation_url, string presentation_id);
+
+    // Called when the frame is ready to process the next state change. Returns
+    // the last session state if it’s changed since the last time the callback
+    // was called. Might cause the event fired with the initial state change.
+    ListenForSessionStateChange()
+        => (PresentationSessionInfo sessionInfo,
+            PresentationSessionState newState);
 };