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

public/platform/modules/presentation/WebPresentationAvailabilityObserver.h

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef WebPresentationAvailabilityObserver_h
+#define WebPresentationAvailabilityObserver_h
+
+namespace blink {
+
+// WebPresentationAvailabilityObserver is an interface that is implemented by
+// objects that wish to be notified when there is a presentation display
+// availability change.
+class WebPresentationAvailabilityObserver {
+public:
+    virtual void availabilityChanged(bool) = 0;
+};
+
+} // namespace blink
+
+#endif // WebPresentationAvailabilityObserver_h