Turn StyleSharing to 11.

In Sky we can style share if our TreeScopes have the same styles, our :host
styles are the same, and we'd inherit the same styles. This allows a lot of
simplification to the style sharing logic since we don't need to deal with
descendant selectors or tree boundary crossing rules:

- We can remove the logic that was checking that we were distributed
to the same insertion points since there's no ::content selectors.

- We can check the actual inherited values instead of looking at the
parentOrSHadowHostNode(). We used to look at the node in Blink because we
were checking that you'd get the same descendant selectors applied. In Sky
we instead want to make sure you'd inherit the same values. This also
means we don't need the element().parentOrShadowHostElement() != parent case
in the SharedStyleFinder which was trying to deal with descendant selectors
again.

I also removed the checks that were redundant with the checks inside
supportsStyleSharing() which we always check before adding sharing
candidates.

Finally by refactoring the code to make the TreeScope style check work it
exposed that the Document::styleSheets() and TreeScope::styleSheets() APIs
are now dead. A future patch will delete the now dead StyleSheetList class
as well.

This change makes the city-list application share between all the items in
the list, and all of the headers of the same type now share as well.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/796713002