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

Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
@@ skipping 289 matching lines @@
       'exported/WebMediaStream.cpp',
       'exported/WebMediaStreamSource.cpp',
       'exported/WebMediaStreamTrack.cpp',
       'exported/WebMediaStreamTrackSourcesRequest.cpp',
       'exported/WebMemoryAllocatorDump.cpp',
       'exported/WebMemoryDumpProvider.cpp',
       'exported/WebMessagePortChannelClient.cpp',
       'exported/WebPasswordCredential.cpp',
       'exported/WebPrerender.cpp',
       'exported/WebPrerenderingSupport.cpp',
+      'exported/WebPresentationAvailabilityObserver.cpp',
       'exported/WebPresentationController.cpp',
       'exported/WebProcessMemoryDump.cpp',
       'exported/WebRTCConfiguration.cpp',
       'exported/WebRTCICECandidate.cpp',
       'exported/WebRTCOfferOptions.cpp',
       'exported/WebRTCSessionDescription.cpp',
       'exported/WebRTCSessionDescriptionRequest.cpp',
       'exported/WebRTCStatsRequest.cpp',
       'exported/WebRTCStatsResponse.cpp',
       'exported/WebRTCVoidRequest.cpp',
@@ skipping 744 matching lines @@
       ['os_posix==1 and OS!="mac"',
         {
           'platform_test_files': [
             'text/LocaleICUTest.cpp',
           ],
         }
       ],
     ],
   },
 }