[PresentationAPI] Added plumbing for start/joinSession from JS to platform

public/platform/modules/presentation/WebPresentationClient.h

Index: public/platform/modules/presentation/WebPresentationClient.h
diff --git a/public/platform/modules/presentation/WebPresentationClient.h b/public/platform/modules/presentation/WebPresentationClient.h
index e09436322a0c58fd706935829288182c088c2fad..13325cd825186a9e0f1010d2ed327d806be520b6 100644
--- a/public/platform/modules/presentation/WebPresentationClient.h
+++ b/public/platform/modules/presentation/WebPresentationClient.h
@@ -5,9 +5,21 @@
 #ifndef WebPresentationClient_h
 #define WebPresentationClient_h
 
+#include "public/platform/WebCallbacks.h"
+#include "public/platform/WebString.h"
+
 namespace blink {
 
 class WebPresentationController;
+class WebPresentationSessionClient;
+class WebString;
+
+struct WebPresentationError;
+
+// If session was created, callback's onSuccess() is invoked with the per-frame unique id of the
+// presentation session interface implementation by the embedder. Otherwise, onError() is invoked
+// with the error code and message.
+using WebPresentationSessionClientCallbacks = WebCallbacks<WebString, WebPresentationError>;
 
 // The implementation the embedder has to provide for the Presentation API to work.
 class WebPresentationClient {
@@ -20,6 +32,14 @@ public:
     // Called when the frame attaches the first event listener to or removes the
     // last event listener from the |availablechange| event.
     virtual void updateAvailableChangeWatched(bool watched) = 0;
+
+    // Called when the frame request to start a new session.
+    // The ownership of the |callbacks| argument is transferred to the embedder.
+    virtual void startSession(const WebString& presentationUrl, const WebString& presentationId, WebPresentationSessionClientCallbacks*) = 0;
+
+    // Called when the frame request to start a new session.
+    // The ownership of the |callbacks| argument is transferred to the embedder.
+    virtual void joinSession(const WebString& presentationUrl, const WebString& presentationId, WebPresentationSessionClientCallbacks*) = 0;
 };
 
 } // namespace blink