Make more [TreatNullAs=NullString] DOMString arguments nullable

Source/core/html/HTMLMediaElement.idl

 /*
  * Copyright (C) 2007, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 28 matching lines @@
 
     const unsigned short NETWORK_EMPTY = 0;
     const unsigned short NETWORK_IDLE = 1;
     const unsigned short NETWORK_LOADING = 2;
     const unsigned short NETWORK_NO_SOURCE = 3;
     readonly attribute unsigned short networkState;
     attribute DOMString preload;
 
     readonly attribute TimeRanges buffered;
     void load();
-    DOMString canPlayType(DOMString type, [Default=Undefined, TreatNullAs=NullString, TreatUndefinedAs=NullString] optional DOMString keySystem);
+    DOMString canPlayType(DOMString type, [Default=Undefined, TreatUndefinedAs=NullString] optional DOMString? keySystem);
 
     // ready state
     const unsigned short HAVE_NOTHING = 0;
     const unsigned short HAVE_METADATA = 1;
     const unsigned short HAVE_CURRENT_DATA = 2;
     const unsigned short HAVE_FUTURE_DATA = 3;
     const unsigned short HAVE_ENOUGH_DATA = 4;
     readonly attribute unsigned short readyState;
     readonly attribute boolean seeking;
 
@@ skipping 28 matching lines @@
     [RaisesException] TextTrack addTextTrack(DOMString kind, optional DOMString label, optional DOMString language);
 
     // WebKit extensions
 
     // The number of bytes consumed by the media decoder.
     [MeasureAs=PrefixedAudioDecodedByteCount] readonly attribute unsigned long webkitAudioDecodedByteCount;
     [MeasureAs=PrefixedVideoDecodedByteCount] readonly attribute unsigned long webkitVideoDecodedByteCount;
 
     [Reflect, RuntimeEnabled=SubresourceIntegrity] attribute DOMString integrity;
 };