Presentation API: ignore page visibility changes if ExecutionContext is destoyed.

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
 
+#include "public/platform/WebCommon.h"
 #include "public/platform/WebURL.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 for a given URL.
-class WebPresentationAvailabilityObserver {
+class BLINK_PLATFORM_EXPORT WebPresentationAvailabilityObserver {
 public:
     virtual ~WebPresentationAvailabilityObserver() = default;
 
     virtual void availabilityChanged(bool) = 0;
 
     virtual const WebURL url() const = 0;
 };
 
 } // namespace blink
 
 #endif // WebPresentationAvailabilityObserver_h