Make more [TreatNullAs=NullString] DOMString arguments nullable

Source/modules/encryptedmedia/HTMLMediaElementEncryptedMedia.idl

Index: Source/modules/encryptedmedia/HTMLMediaElementEncryptedMedia.idl
diff --git a/Source/modules/encryptedmedia/HTMLMediaElementEncryptedMedia.idl b/Source/modules/encryptedmedia/HTMLMediaElementEncryptedMedia.idl
index 067f11a1829e7c5145884291ac8536fb1df979ef..ed9419904e7b0a491e41bf8b2087261f650adc8e 100644
--- a/Source/modules/encryptedmedia/HTMLMediaElementEncryptedMedia.idl
+++ b/Source/modules/encryptedmedia/HTMLMediaElementEncryptedMedia.idl
@@ -3,9 +3,9 @@
 // found in the LICENSE file.
 
 partial interface HTMLMediaElement {
-    [RuntimeEnabled=PrefixedEncryptedMedia, RaisesException, MeasureAs=PrefixedMediaGenerateKeyRequest] void webkitGenerateKeyRequest([TreatNullAs=NullString, TreatUndefinedAs=NullString] DOMString keySystem, optional Uint8Array initData);
-    [RuntimeEnabled=PrefixedEncryptedMedia, RaisesException, MeasureAs=PrefixedMediaAddKey] void webkitAddKey([TreatNullAs=NullString, TreatUndefinedAs=NullString] DOMString keySystem, Uint8Array key, optional Uint8Array initData, optional DOMString sessionId = null);
-    [RuntimeEnabled=PrefixedEncryptedMedia, RaisesException, MeasureAs=PrefixedMediaCancelKeyRequest] void webkitCancelKeyRequest([TreatNullAs=NullString, TreatUndefinedAs=NullString] DOMString keySystem, optional DOMString sessionId = null);
+    [RuntimeEnabled=PrefixedEncryptedMedia, RaisesException, MeasureAs=PrefixedMediaGenerateKeyRequest] void webkitGenerateKeyRequest([TreatUndefinedAs=NullString] DOMString? keySystem, optional Uint8Array initData);
+    [RuntimeEnabled=PrefixedEncryptedMedia, RaisesException, MeasureAs=PrefixedMediaAddKey] void webkitAddKey([TreatUndefinedAs=NullString] DOMString? keySystem, Uint8Array key, optional Uint8Array initData, optional DOMString sessionId = null);
+    [RuntimeEnabled=PrefixedEncryptedMedia, RaisesException, MeasureAs=PrefixedMediaCancelKeyRequest] void webkitCancelKeyRequest([TreatUndefinedAs=NullString] DOMString? keySystem, optional DOMString sessionId = null);
 
     [RuntimeEnabled=PrefixedEncryptedMedia] attribute EventHandler onwebkitkeyadded;
     [RuntimeEnabled=PrefixedEncryptedMedia] attribute EventHandler onwebkitkeyerror;