style: Make Gecko's media query evaluation work with a document, not a pres context.
style: Make Gecko's media query evaluation work with a document, not a pres context.

Returning a zero-sized viewport in the case there's no shell / pres context.

For now, no other change yet. After this we can start tweaking the ShadowRoot
Stylist setup, and even try to return computed styles without a document \o/.

Bug: 1430608
MozReview-Commit-ID: 3cT2PKQISri

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