[PresentationAPI] Implementing start/joinSession from WebPresentationClient to PresentationService.

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";
+
 interface PresentationService {
     // Returns the last screen availability state if it’s changed since the last
     // time the method was called. The client has to call this method again when
     // handling the result (provided via Mojo callback) to get the next update
     // about the availability status.
     // 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 startSession() is called by the frame. The result callback
+    // will return true if starting the session succeeded (so the session object
+    // is valid) or false with an error message if starting the session failed.
+    // 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 any page
+    // can join the session later on.
+    StartSession(
+            string presentation_url,
+            string? presentation_id) => (
+                    bool success,

[mlamouri (slow - plz ping), 2015/02/18 15:16:43]

Maybe the following would match style better:

StartSession(string presentation_url, string? presentation_id)
  => (bool success, PresentationSession? session, string? error, string? url,
string? id);

[whywhat, 2015/02/19 16:32:22]

Done.

+                    PresentationSession&? session,
+                    string? error,
+                    string? url,
+                    string? id);

[mlamouri (slow - plz ping), 2015/02/18 15:16:41]

Maybe you could have something more generic here? Like a struct carrying some
data. For example url and id. Maybe event PresentationSession if they can carry
interfaces. The error should probably stay out of that struct. The struct would
be null if there is an error. I would remove the success boolean because it
should be easy for anyone to check the returned value in order to see if it was
a success.

[whywhat, 2015/02/19 16:32:22]

Done.

+
+    // Called when joinSession() is called by the frame. The result callback
+    // works the same as for the method above. JoinSession is used to silently
+    // (no user action needed) rejoin the known session (i.e. when the page

[mlamouri (slow - plz ping), 2015/02/18 15:16:42]

s/the/a/ ?

[whywhat, 2015/02/19 16:32:22]

Done.

+    // navigates or the user opens another tab).
+    JoinSession(
+            string presentation_url,
+            string? presentation_id) => (
+                    bool success,
+                    PresentationSession&? session,
+                    string? url,
+                    string? id,
+                    string? error);

[mlamouri (slow - plz ping), 2015/02/18 15:16:43]

ditto.

[whywhat, 2015/02/19 16:32:22]

Done.

 };