Drop support for [TreatUndefinedAs=NullString]

third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.idl

Index: third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.idl
diff --git a/third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.idl b/third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.idl
index 9f204dcefb790b91017e13c6b037609661aa7914..2e48d3ee8ebb1883df9c06a7b7a5a0cc86617bbb 100644
--- a/third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.idl
+++ b/third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.idl
@@ -112,7 +112,7 @@ enum RTCIceConnectionState {
     // https://w3c.github.io/webrtc-pc/#peer-to-peer-data-api
     // TODO(guidou): The label argument should have [TreatNullAs=EmptyString]
     // and be non-nullable.
-    [CallWith=ExecutionContext, RaisesException] RTCDataChannel createDataChannel([TreatUndefinedAs=NullString] DOMString? label, optional Dictionary options);
+    [CallWith=ExecutionContext, RaisesException] RTCDataChannel createDataChannel(DOMString? label, optional Dictionary options);
     attribute EventHandler ondatachannel;
 
     // Certificate management