[w3c/webcomponents] Allow customizing base of a shadow root (#581)

It'd be very convenient in a number of applications to be able to customize base in shadow root, either via `<base>` tag or `baseURI` property. Shadow root are now used for embeds (e.g. Twitter embed) and it'd be good to have relative URL resolution scoped to shadow root.

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https://github.com/w3c/webcomponents/issues/581