[FileAPI] Don't use [EnsureUTF16] (gone from Web IDL) (#9)

It is used in https://w3c.github.io/FileAPI/#file for the `fileName` argument in the constructor. It has however been removed from the Web IDL spec:
https://github.com/heycam/webidl/commit/e2cf800ef401578db8d1445d815dc13bc8899554

Perhaps USVString would work, that's what it already is in Gecko:
https://github.com/mozilla/gecko-dev/blob/9d30f59efd1f82cfa80633c43834b7f08a9c77e1/dom/webidl/File.webidl

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/FileAPI/issues/9