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

Source/modules/presentation/PresentationAvailability.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 PresentationAvailability_h
+#define PresentationAvailability_h
+
+#include "core/dom/ActiveDOMObject.h"
+#include "core/events/EventTarget.h"
+#include "public/platform/modules/presentation/WebPresentationAvailabilityObserver.h"
+
+namespace blink {
+
+class ExecutionContext;
+class ScriptPromiseResolver;
+
+// Expose whether there is a presentation display available. The object will be
+// initialized with a default value passed via ::take() and will then subscribe
+// to receive callbacks if the status were to change. The object will only
+// listen to changes when required.
+class PresentationAvailability final
+    : public RefCountedGarbageCollectedEventTargetWithInlineData<PresentationAvailability>
+    , public ActiveDOMObject
+    , public WebPresentationAvailabilityObserver {
+    REFCOUNTED_GARBAGE_COLLECTED_EVENT_TARGET(PresentationAvailability);
+    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(PresentationAvailability);
+    DEFINE_WRAPPERTYPEINFO();
+public:
+    static PresentationAvailability* take(ScriptPromiseResolver*, bool);
+
+    static PresentationAvailability* create(ExecutionContext*, bool);
+    ~PresentationAvailability() override;
+
+    // EventTarget implementation.
+    const AtomicString& interfaceName() const override;
+    ExecutionContext* executionContext() const override;
+
+    // WebPresentationAvailabilityObserver implementation.
+    void availabilityChanged(bool) override;
+
+    // ActiveDOMObject implementation.
+    bool hasPendingActivity() const override;
+    void suspend() override;
+    void resume() override;
+    void stop() override;
+
+    bool value() const;
+
+    DEFINE_ATTRIBUTE_EVENT_LISTENER(change);
+
+    DECLARE_VIRTUAL_TRACE();
+
+private:
+    PresentationAvailability(ExecutionContext*, bool);
+
+    void startListening();
+    void stopListening();
+
+    bool m_value;
+    bool m_listening;
+};
+
+} // namespace blink
+
+#endif // PresentationAvailability_h