[w3c/webcomponents] Is <slot> a replaced element or not? (#564)

[Per the WHATWG HTML Spec](https://html.spec.whatwg.org/multipage/rendering.html#flow-content-3), the `<slot>` element is listed in the section "Non-replaced elements". However, given its presentation hint (`display: contents`), it seems that `<slot>` might better be treated as a replaced element. Also Chrome 53+ seems have implemented it as such (`::before` and `::after` selectors no longer work on `<slot>`)

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