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);
 
   // 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);
 
   // 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 {
+  // Called when the client tries to listen for screen availability changes but

[whywhat, 2015/07/06 18:27:18]

nit: Rephrase as

"Called when the client starts listening for screen availability and passive
availability monitoring is not supported by the device either permanently
(hardware limitations) or temporarily (battery saver mode)."

Should we indicate if the limitation is temporarily or not so the page/Blink
layer could try again depending on battery status, for example?

+  // 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

[whywhat, 2015/07/06 18:27:18]

nit: "Called immediately when the client starts listening for screen
availability if the current availability state is known and later to notify the
client of the state updates."

+  // 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);
 };