MSE: Implement TrackDefault 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 69 matching lines @@
       'indexeddb/IDBIndex.idl',
       'indexeddb/IDBKeyRange.idl',
       'indexeddb/IDBObjectStore.idl',
       '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/VideoPlaybackQuality.idl',
       'mediastream/MediaDeviceInfo.idl',
       'mediastream/MediaDeviceInfoCallback.idl',
       'mediastream/MediaStream.idl',
       'mediastream/MediaStreamEvent.idl',
       'mediastream/MediaStreamTrack.idl',
       'mediastream/MediaStreamTrackEvent.idl',
       'mediastream/MediaStreamTrackSourcesCallback.idl',
       'mediastream/NavigatorUserMediaError.idl',
       'mediastream/NavigatorUserMediaErrorCallback.idl',
@@ skipping 556 matching lines @@
       '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',
+      'mediasource/TrackDefault.cpp',
+      'mediasource/TrackDefault.h',
       'mediasource/URLMediaSource.cpp',
       'mediasource/URLMediaSource.h',
       'mediasource/VideoPlaybackQuality.cpp',
       'mediasource/VideoPlaybackQuality.h',
       'mediastream/MediaConstraintsImpl.cpp',
       'mediastream/MediaConstraintsImpl.h',
       'mediastream/MediaDeviceInfo.cpp',
       'mediastream/MediaDeviceInfo.h',
       'mediastream/MediaDeviceInfoCallback.h',
       'mediastream/MediaDevicesRequest.cpp',
@@ skipping 443 matching lines @@
       'serviceworkers/CacheTest.cpp',
       'serviceworkers/FetchResponseDataTest.cpp',
       'serviceworkers/RequestTest.cpp',
       'serviceworkers/ResponseTest.cpp',
       'serviceworkers/ServiceWorkerContainerTest.cpp',
       'websockets/DOMWebSocketTest.cpp',
       'websockets/DocumentWebSocketChannelTest.cpp',
     ],
   },
 }