[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";
+
+struct PresentationSessionInfo {
+    PresentationSession session;
+    string url;
+    string id;
+};
+
+enum PresentationErrorType {
+    NO_AVAILABLE_SCREENS,
+    SESSION_REQUEST_CANCELLED,
+    NO_PRESENTATION_FOUND,
+    UNKNOWN,
+};
+
+struct PresentationError {
+    PresentationErrorType errorType;
+    string message;
+};
+
 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 a non-null PresentationSessionInfo if starting the session
+    // succeeded (so the session object is valid) or an error message if

[mark a. foltz, 2015/02/26 00:13:42]

I would say:

will return a non-null and valid PresentationSessionInfo if starting the session
succeeded, or null with a PresentationError if starting the session failed.

[whywhat, 2015/02/26 16:15:07]

Done.

+    // 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 is used to silently

[mark a. foltz, 2015/02/26 00:13:42]

JoinSession will join a known session (i.e. ...) silently and without user
action.

[whywhat, 2015/02/26 16:15:07]

Done.

+    // (no user action needed) join a known session (i.e. when the page
+    // navigates or the user opens another tab).
+    JoinSession(string presentation_url, string? presentation_id)
+        => (PresentationSessionInfo? sessionInfo, PresentationError? error);
 };