[webnfc] Use BufferSource in ndef_record_init.idl

Remove the TODO as the union type has been supported in the bindings
generator.

BUG=520391

Change-Id: I709726c9378b739944eebf26ff9b1ccb0b8cd823
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2093397Reviewed-by: Rijubrata Bhaumik <rijubrata.bhaumik@intel.com>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Donna Wu <donna.wu@intel.com>
Cr-Commit-Position: refs/heads/master@{#749075}

third_party/blink/renderer/modules/nfc/ndef_record_init.idl

@@ -4,11 +4,7 @@
 
 // https://w3c.github.io/web-nfc/#idl-def-ndefrecorddatasource
 
-// The bindings generator does not support "typedef in union".
-// Must be (DOMString or BufferSource or NDEFMessageInit).
-// TODO(http://crbug.com/1019126): Support nested typedef in Web IDLs.
-typedef (DOMString or ArrayBuffer or ArrayBufferView or NDEFMessageInit)
-  NDEFRecordDataSource;
+typedef (DOMString or BufferSource or NDEFMessageInit) NDEFRecordDataSource;
 
 // https://w3c.github.io/web-nfc/#dom-ndefrecordinit