Add stub for assigning media session to media elements

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 258 matching lines @@
       'filesystem/HTMLInputElementFileSystem.idl',
       'filesystem/WindowFileSystem.idl',
       'filesystem/WorkerGlobalScopeFileSystem.idl',
       'gamepad/NavigatorGamepad.idl',
       'geofencing/ServiceWorkerGlobalScopeGeofencing.idl',
       'geofencing/ServiceWorkerRegistrationGeofencing.idl',
       'geofencing/WorkerNavigatorGeofencing.idl',
       'geolocation/NavigatorGeolocation.idl',
       'indexeddb/WindowIndexedDatabase.idl',
       'indexeddb/WorkerGlobalScopeIndexedDatabase.idl',
+      'mediasession/HTMLMediaElementMediaSession.idl',
       'mediasource/HTMLVideoElementMediaSource.idl',
       'mediasource/URLMediaSource.idl',
       'mediastream/NavigatorMediaStream.idl',
       'mediastream/NavigatorUserMedia.idl',
       'mediastream/URLMediaStream.idl',
       'mediastream/WindowMediaStream.idl',
       'navigatorconnect/NavigatorServices.idl',
       'navigatorconnect/ServiceWorkerGlobalScopeNavigatorConnect.idl',
       'navigatorconnect/WorkerNavigatorServices.idl',
       'navigatorcontentutils/NavigatorContentUtils.idl',
@@ skipping 640 matching lines @@
       'indexeddb/IndexedDBClient.cpp',
       'indexeddb/IndexedDBClient.h',
       'indexeddb/InspectorIndexedDBAgent.cpp',
       'indexeddb/InspectorIndexedDBAgent.h',
       'indexeddb/WebIDBCallbacksImpl.cpp',
       'indexeddb/WebIDBCallbacksImpl.h',
       'indexeddb/WebIDBDatabaseCallbacksImpl.cpp',
       'indexeddb/WebIDBDatabaseCallbacksImpl.h',
       'indexeddb/WorkerGlobalScopeIndexedDatabase.cpp',
       'indexeddb/WorkerGlobalScopeIndexedDatabase.h',
+      'mediasession/HTMLMediaElementMediaSession.cpp',
+      'mediasession/HTMLMediaElementMediaSession.h',
+      'mediasession/MediaSession.cpp',
       'mediasession/MediaSession.h',
-      'mediasession/MediaSession.cpp',
       '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 553 matching lines @@
       'webaudio/AudioBasicProcessorHandlerTest.cpp',
       'webaudio/ConvolverNodeTest.cpp',
       'webaudio/DynamicsCompressorNodeTest.cpp',
       'webaudio/ScriptProcessorNodeTest.cpp',
       'webaudio/StereoPannerNodeTest.cpp',
       'websockets/DOMWebSocketTest.cpp',
       'websockets/DocumentWebSocketChannelTest.cpp',
     ],
   },
 }