NOT FOR LANDING Introduce and use WebMediaSession

Source/modules/modules.gypi

 {
   'includes': [
     '../core/core_generated.gypi',
     'modules_generated.gypi',
   ],
   'variables': {
     # Experimental hooks for embedder to provide extra IDL and source files.
     #
     # Note: this is not a supported API. If you rely on this, you will be broken
     # from time to time as the code generator changes in backward incompatible
@@ skipping 1076 matching lines @@
       'indexeddb/WorkerGlobalScopeIndexedDatabase.cpp',
       'indexeddb/WorkerGlobalScopeIndexedDatabase.h',
       'mediarecorder/MediaRecorder.cpp',
       'mediarecorder/MediaRecorder.h',
       'mediarecorder/MediaRecorderErrorEvent.cpp',
       'mediarecorder/MediaRecorderErrorEvent.h',
       'mediasession/HTMLMediaElementMediaSession.cpp',
       'mediasession/HTMLMediaElementMediaSession.h',
       'mediasession/MediaSession.cpp',
       'mediasession/MediaSession.h',
+      'mediasession/MediaSessionError.cpp',
+      'mediasession/MediaSessionError.h',
       'mediasource/HTMLVideoElementMediaSource.cpp',
       'mediasource/HTMLVideoElementMediaSource.h',
       'mediasource/MediaSource.cpp',
       'mediasource/MediaSource.h',
       'mediasource/MediaSourceRegistry.cpp',
       'mediasource/MediaSourceRegistry.h',
       'mediasource/SourceBuffer.cpp',
       'mediasource/SourceBuffer.h',
       'mediasource/SourceBufferList.cpp',
       'mediasource/SourceBufferList.h',
@@ skipping 699 matching lines @@
       'fetch/FetchDataLoaderTest.cpp',
       'fetch/FetchFormDataConsumerHandleTest.cpp',
       'fetch/FetchResponseDataTest.cpp',
       'fetch/RequestTest.cpp',
       'fetch/ResponseTest.cpp',
       'filesystem/DOMFileSystemBaseTest.cpp',
       'imagebitmap/ImageBitmapModuleTest.cpp',
       'indexeddb/IDBKeyPathTest.cpp',
       'indexeddb/IDBRequestTest.cpp',
       'indexeddb/IDBTransactionTest.cpp',
+      'mediasession/MediaSessionTest.cpp',
       'mediastream/RTCDataChannelTest.cpp',
       'notifications/NotificationDataTest.cpp',
       'presentation/PresentationAvailabilityTest.cpp',
       'serviceworkers/ServiceWorkerContainerTest.cpp',
       'webaudio/AudioBasicProcessorHandlerTest.cpp',
       'webaudio/ConvolverNodeTest.cpp',
       'webaudio/DynamicsCompressorNodeTest.cpp',
       'webaudio/ScriptProcessorNodeTest.cpp',
       'webaudio/StereoPannerNodeTest.cpp',
       'websockets/DOMWebSocketTest.cpp',
       'websockets/DocumentWebSocketChannelTest.cpp',
     ],
   },
 }