Decouple MIDIAccess initialization from MIDIAccess class.

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': [],
     # Files for which bindings (.cpp and .h files) will be generated
@@ skipping 866 matching lines @@
       'webdatabase/sqlite/SQLiteFileSystem.cpp',
       'webdatabase/sqlite/SQLiteFileSystem.h',
       'webdatabase/sqlite/SQLiteFileSystemPosix.cpp',
       'webdatabase/sqlite/SQLiteFileSystemWin.cpp',
       'webdatabase/sqlite/SQLiteStatement.cpp',
       'webdatabase/sqlite/SQLiteStatement.h',
       'webdatabase/sqlite/SQLiteTransaction.cpp',
       'webdatabase/sqlite/SQLiteTransaction.h',
       'webmidi/MIDIAccess.cpp',
       'webmidi/MIDIAccess.h',
+      'webmidi/MIDIAccessInitializer.cpp',
+      'webmidi/MIDIAccessInitializer.h',
       'webmidi/MIDIAccessor.cpp',
       'webmidi/MIDIAccessor.h',
       'webmidi/MIDIAccessorClient.h',
       'webmidi/MIDIClient.h',
       'webmidi/MIDIClientMock.cpp',
       'webmidi/MIDIClientMock.h',
       'webmidi/MIDIConnectionEvent.h',
       'webmidi/MIDIController.cpp',
       'webmidi/MIDIController.h',
       'webmidi/MIDIErrorCallback.h',
@@ skipping 61 matching lines @@
       'indexeddb/IDBRequestTest.cpp',
       'indexeddb/IDBTransactionTest.cpp',
       'websockets/WebSocketDeflaterTest.cpp',
       'websockets/WebSocketExtensionDispatcherTest.cpp',
       'websockets/WebSocketExtensionParserTest.cpp',
       'websockets/WebSocketPerMessageDeflateTest.cpp',
       'websockets/WebSocketTest.cpp',
     ],
   },
 }