[Presentation API] Plumbing the |defaultpresentationstart| event from Blink to browser/Mojo.

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;
 
 import "presentation_session.mojom";
 
 struct PresentationSessionInfo {
     PresentationSession session;
@@ skipping 21 matching lines @@
     // May start discovery of the presentation screens. The implementation might
     // stop discovery once there are no active calls to GetScreenAvailability.
     // |presentation_url| can be specified to help the implementation to filter
     // out incompatible screens.
     GetScreenAvailability(string? presentation_url) => (bool available);
 
     // Called when the frame no longer listens to the
     // |availablechange| event.
     OnScreenAvailabilityListenerRemoved();
 
+    // Called when the page is ready to listen to the next such event. Indicates
+    // the frame is ready to handle the default presentation started by the user
+    // agent.

[mark a. foltz, 2015/03/05 19:53:51]

Presumably this places a requirement on the implementation to queue up any such
session before this is called?  Can this requirement be described explicitly?

+    ListenForDefaultPresentationStart()
+        => (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
     // UA will generate the presentation id.
     StartSession(string presentation_url, string? presentation_id)
          => (PresentationSessionInfo? sessionInfo, PresentationError? error);
 
     // 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);
 };