[Presentation API] Implement ondefaultsessionstart in PSImpl.

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 34 matching lines @@
         (string? presentation_url, bool available);
 
     // Called when the frame no longer listens to the |availablechange| event.
     RemoveScreenAvailabilityListener(string? presentation_url);
 
     // Called when the renderer is ready to receive the browser initiated
     // session. If the default session is started by the embedder before this
     // call, the embedder may queue it and run the callback when the call is
     // performed.
     ListenForDefaultSessionStart()
-        => (PresentationSessionInfo defaultSessionInfo);
+        => (PresentationSessionInfo? defaultSessionInfo);
 
     // Called when startSession() is called by the frame. The result callback
     // will return a non-null and valid PresentationSessionInfo if starting the
     // session succeeded, or null with a PresentationError if starting the
     // session failed.
     // The presentation id is always returned along with the initialized
     // session on success.
     // If the UA identifies a matching session (same presentation url and id),
     // the user may choose this existing session and the page will join it
     // rather than get a new one. An empty presentation id means that the
@@ skipping 11 matching lines @@
     // 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);
 };