Re: [webcomponents] attachShadow must throw a TypeError when mode is omitted (#311)

Yeah, I just re-read, and I was right the first time. Without `required`, if it is `undefined` (e.g. via omission from the JS object that gets converted into an IDL dictionary), the conversion to ShadowRootMode will not take place.

So yeah, just add `required` and it'll work as expected.

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