[whatwg/dom] Use mixins in IDL definitions (#529)

[Mixins](https://heycam.github.io/webidl/#idl-interface-mixins) have been added to Web IDL to replace the [[NoInterfaceObject](https://heycam.github.io/webidl/#NoInterfaceObject)] extended attribute + `implements` statement.

See https://github.com/heycam/webidl/pull/433.

/cc @tobie

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