Add minimal implementation of unprefixed MediaSource API that has feature parity with prefixed API

Source/modules/modules.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
     # ways.
     'extra_blink_module_idl_files': [],
     'extra_blink_module_files': [],
 
@@ skipping 54 matching lines @@
       'indexeddb/IDBFactory.idl',
       'indexeddb/IDBIndex.idl',
       'indexeddb/IDBKeyRange.idl',
       'indexeddb/IDBObjectStore.idl',
       'indexeddb/IDBOpenDBRequest.idl',
       'indexeddb/IDBRequest.idl',
       'indexeddb/IDBTransaction.idl',
       'indexeddb/IDBVersionChangeEvent.idl',
       'indexeddb/WorkerContextIndexedDatabase.idl',
       'mediasource/DOMWindowMediaSource.idl',
+      'mediasource/MediaSource.idl',
+      'mediasource/SourceBuffer.idl',
+      'mediasource/SourceBufferList.idl',
       'mediasource/WebKitMediaSource.idl',
       'mediasource/WebKitSourceBuffer.idl',
       'mediasource/WebKitSourceBufferList.idl',
       'mediastream/MediaStream.idl',
       'mediastream/MediaStreamEvent.idl',
       'mediastream/MediaStreamTrack.idl',
       'mediastream/MediaStreamTrackEvent.idl',
       'mediastream/NavigatorMediaStream.idl',
       'mediastream/NavigatorUserMediaError.idl',
       'mediastream/NavigatorUserMediaErrorCallback.idl',
@@ skipping 251 matching lines @@
       'indexeddb/IDBTransaction.h',
       'indexeddb/IDBVersionChangeEvent.cpp',
       'indexeddb/IDBVersionChangeEvent.h',
       'indexeddb/IndexedDB.h',
       'indexeddb/PageGroupIndexedDatabase.cpp',
       'indexeddb/PageGroupIndexedDatabase.h',
       'indexeddb/WorkerContextIndexedDatabase.cpp',
       'indexeddb/WorkerContextIndexedDatabase.h',
       'indexeddb/chromium/IDBFactoryBackendInterfaceChromium.cpp',
       'indexeddb/chromium/IDBFactoryBackendInterfaceChromium.h',
+      'mediasource/MediaSource.cpp',
+      'mediasource/MediaSource.h',
       'mediasource/MediaSourceBase.cpp',
       'mediasource/MediaSourceBase.h',
       'mediasource/MediaSourceRegistry.cpp',
       'mediasource/MediaSourceRegistry.h',
+      'mediasource/SourceBuffer.cpp',
+      'mediasource/SourceBuffer.h',
+      'mediasource/SourceBufferList.cpp',
+      'mediasource/SourceBufferList.h',
       'mediasource/WebKitMediaSource.cpp',
       'mediasource/WebKitMediaSource.h',
       'mediasource/WebKitSourceBuffer.cpp',
       'mediasource/WebKitSourceBuffer.h',
       'mediasource/WebKitSourceBufferList.cpp',
       'mediasource/WebKitSourceBufferList.h',
       'mediastream/MediaConstraintsImpl.cpp',
       'mediastream/MediaConstraintsImpl.h',
       'mediastream/MediaStream.cpp',
       'mediastream/MediaStream.h',
@@ skipping 309 matching lines @@
       'websockets/WorkerThreadableWebSocketChannel.cpp',
       'websockets/WorkerThreadableWebSocketChannel.h',
     ],
     'modules_unittest_files': [
       'websockets/WebSocketDeflaterTest.cpp',
       'websockets/WebSocketExtensionDispatcherTest.cpp',
       'websockets/WebSocketPerMessageDeflateTest.cpp',
     ],
   },
 }