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

content/common/presentation/presentation_service.mojom

Index: content/common/presentation/presentation_service.mojom
diff --git a/content/common/presentation/presentation_service.mojom b/content/common/presentation/presentation_service.mojom
index 2ff381a41cb5d2f7affb681cf2cc65dfddadd909..774fe30787824f00e44c21b3bc0272b777efedeb 100644
--- a/content/common/presentation/presentation_service.mojom
+++ b/content/common/presentation/presentation_service.mojom
@@ -4,6 +4,8 @@
 
 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
@@ -18,4 +20,33 @@ interface PresentationService {
     // 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.

 };