[css-scoping] Remove shadow-piercing and multiple shadow roots, and the change in Order of Appearance

WebApps WG resolved to remove shadow-piercing and multiple shadow roots[1].

As a consequence of this change, two declarations in different trees should
always be resolved by the Shadow Tree criteria and should not require
calculating Order of Appearance, if I understand correctly.

If I'm missing cases where we still need Order of Appearance in tree of
trees, I'd appreciate to be pointed out.

Tab, could you make the change, or PR[2] is on its way if you prefer.

[1]
https://lists.w3.org/Archives/Public/public-webapps/2015AprJun/att-0307/24-minutes.html#ResolutionSummary
[2] https://github.com/w3c/csswg-drafts/pull/21/files

/koji