[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..a82ba591b95bdd2f34ddd4dd68dc6a8a72653088 100644
--- a/content/common/presentation/presentation_service.mojom
+++ b/content/common/presentation/presentation_service.mojom
@@ -4,6 +4,19 @@
 
 module presentation;
 
+import "presentation_session.mojom";
+
+struct PresentationSessionInfo {
+    PresentationSession session;

[mark a. foltz, 2015/02/19 22:59:57]

Are |url| and |id| properties of |session|?  Do we need the *Info wrapper
struct?

[whywhat, 2015/02/19 23:22:18]

[Mojo]PresentationSession is an interface meaning it can only contain async
methods, not sync properties like structs.
We could I guess add GetUrl/GetId to the interface and call it once we get
OnCreateSession callback before resolving the Promise - we need to have the id
as it's a readonly sync property of the [W3C]PresentationSession spec. I thought
it would be easier to pass
the values once in the OnCreateSession callback and wrapping it in a struct was
suggested by other reviewers.

I assumed that id will not change during session lifetime. Url is not actually
needed by the spec but might be useful if we want to match sessions on the
renderer side.
Since we have an async

+    string url;
+    string id;
+};
+
+struct PresentationError {
+    int32 errorCode;
+    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
@@ -18,4 +31,24 @@ 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 a non-null PresentationSessionInfo if starting the session
+    // succeeded (so the session object is valid) or an error message 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 is used to silently
+    // (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);
 };