Add MediaSession runtime flag along with stub MediaSession object

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 89 matching lines @@
       'indexeddb/IDBOpenDBRequest.idl',
       'indexeddb/IDBRequest.idl',
       'indexeddb/IDBTransaction.idl',
       'indexeddb/IDBVersionChangeEvent.idl',
       'mediasource/MediaSource.idl',
       'mediasource/SourceBuffer.idl',
       'mediasource/SourceBufferList.idl',
       'mediasource/TrackDefault.idl',
       'mediasource/TrackDefaultList.idl',
       'mediasource/VideoPlaybackQuality.idl',
+      'mediasession/MediaSession.idl',
       'mediastream/MediaDeviceInfo.idl',
       'mediastream/MediaDevices.idl',
       'mediastream/MediaStream.idl',
       'mediastream/MediaStreamEvent.idl',
       'mediastream/MediaStreamTrack.idl',
       'mediastream/MediaStreamTrackEvent.idl',
       'mediastream/MediaStreamTrackSourcesCallback.idl',
       'mediastream/NavigatorUserMediaError.idl',
       'mediastream/NavigatorUserMediaErrorCallback.idl',
       'mediastream/NavigatorUserMediaSuccessCallback.idl',
@@ skipping 792 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/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 543 matching lines @@
       'webaudio/AudioBasicProcessorHandlerTest.cpp',
       'webaudio/ConvolverNodeTest.cpp',
       'webaudio/DynamicsCompressorNodeTest.cpp',
       'webaudio/ScriptProcessorNodeTest.cpp',
       'webaudio/StereoPannerNodeTest.cpp',
       'websockets/DOMWebSocketTest.cpp',
       'websockets/DocumentWebSocketChannelTest.cpp',
     ],
   },
 }