Need to define how to extract the sheets from adoptedStylesheets

[bzbarsky]

https://wicg.github.io/construct-stylesheets/#ref-for-adopted-stylesheets%E2%91%A1 says:

The user agent must include all style sheets in the DocumentOrShadowRoot's adopted stylesheets inside its document or shadow root CSS style sheets.

But "adopted stylesheets" is a JS Array object. The spec needs to define exactly how to extract the sheets from it, because some of the ways of doing that have side-effects (and others may not right now but could grow them in the future).

@tabatkins @domenic