Presentation API: implement .getAvalability() (Blink side).

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 8aaf55c7bff88ded31526a3552f77db9f3847503..40f70a53d1d0dfa6b238204f24b9dc1e742d2f02 100644
--- a/Source/modules/presentation/Presentation.h
+++ b/Source/modules/presentation/Presentation.h
@@ -46,21 +46,10 @@ public:
 
     ScriptPromise startSession(ScriptState*, const String& presentationUrl, const String& presentationId);
     ScriptPromise joinSession(ScriptState*, const String& presentationUrl, const String& presentationId);
+    ScriptPromise getAvailability(ScriptState*, const String& presentationUrl);
 
-    DEFINE_ATTRIBUTE_EVENT_LISTENER(availablechange);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(defaultsessionstart);
 
-    // The embedder needs to keep track if anything is listening to the event so it could stop the
-    // might be expensive screen discovery process.
-    virtual bool addEventListener(const AtomicString& eventType, PassRefPtr<EventListener>, bool useCapture) override;
-    virtual bool removeEventListener(const AtomicString& eventType, PassRefPtr<EventListener>, bool useCapture) override;
-    virtual void removeAllEventListeners() override;
-
-    // Called when the |availablechange| event needs to be fired.
-    void didChangeAvailability(bool available);
-    // Queried by the controller if |availablechange| event has any listeners.
-    bool isAvailableChangeWatched() const;
-
     // Called when the |defaultsessionstart| event needs to be fired.
     void didStartDefaultSession(PresentationSession*);