[PresentationAPI] Plumbing |onstatechange| event for the PresentationSession from Blink to platform/

Source/modules/presentation/Presentation.h

Index: Source/modules/presentation/Presentation.h
diff --git a/Source/modules/presentation/Presentation.h b/Source/modules/presentation/Presentation.h
index 8a5ce56b7ffacd79e69c2b1c3bbe3ed1e235e886..ab92d860cda2a51f53d85273a899bed966a35689 100644
--- a/Source/modules/presentation/Presentation.h
+++ b/Source/modules/presentation/Presentation.h
@@ -21,6 +21,7 @@ namespace blink {
 class LocalFrame;
 class PresentationController;
 class ScriptState;
+class WebPresentationSessionClient;
 
 // Implements the main entry point of the Presentation API corresponding to the Presentation.idl
 // See https://w3c.github.io/presentation-api/#navigatorpresentation for details.
@@ -65,6 +66,9 @@ public:
     // Adds a session to the open sessions list.
     void registerSession(PresentationSession*);
 
+    // Returns the session that matches the WebPresentationSessionClient or null.
+    PresentationSession* findSession(WebPresentationSessionClient*);
+
 private:
     explicit Presentation(LocalFrame*);