Presentation API: allow the Mojo service to reject a getAvailability() call.

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;
 };
@@ skipping 77 matching lines @@
   // silently and without user action.
   JoinSession(string presentation_url, string? presentation_id)
       => (PresentationSessionInfo? sessionInfo, PresentationError? error);
 
   // Called when send() is called by the frame. The true in the
   // result callback notifies that the service is ready for next message.
   // The false in the result callback notifies the renderer to stop sending
   // the send requests and invalidate all pending requests. This occurs
   // for eg., when frame is deleted or navigated away.
   SendSessionMessage(SessionMessage message_request) => (bool success);
-  
+
   // Called when closeSession() is called by the frame.
   CloseSession(string presentation_url, string presentation_id);
 
   // Starts listening for state changes for sessions created on this frame.
   // When state change occurs, PresentationServiceClient::OnSessionStateChanged
   // will be invoked with the session and its new state.
   ListenForSessionStateChange();
 
   // Called when the frame is ready to process the next batch of messages.
   // When the callback carries null messages, there is an error
   // at the presentation service side.
   ListenForSessionMessages()
       => (array<SessionMessage>? messages);
 };
 
 interface PresentationServiceClient {
-  // See PresentationSerivce::ListenForScreenAvailability.
+  // Called when the client tries to listen for screen availability changes but
+  // it is not supported by the device or underlying platform. This can also be
+  // called if the device is currently in a mode where it can't do screen
+  // discoveries (eg. low battery).
+  OnScreenAvailabilityNotSupported();
+
+  // Called when the client is listening for screen availability and the state
+  // changes. When the client starts to listen for screen availability, this
+  // method will always be called to give the current known state. It will then
+  // be called to notify of state updates.
   OnScreenAvailabilityUpdated(bool available);
 
   // See PresentationService::ListenForSessionStateChange.
   OnSessionStateChanged(PresentationSessionInfo sessionInfo,
                         PresentationSessionState newState);
 };