Make right-most compound check explicit.

Make right-most compound check explicit.

We set SelectorCheckingContext::elementStyle to null to indicate that we
were not in the rightmost compound selector. Use a flag in the context
instead to make it clearer what we mean.

An alternative would be to have a method inRightmostCompound() on the
context instead that checks the elementStyle for nullptr.

R=esprehn@chromium.org,timloh@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=195606

[esprehn, 2015-05-20 00:34:37]

I'm not a huge fan of having so many state bits, this means during selector
checking there's tons of assignment churn as we create a new context for every
relation. I'm trying to undo that currently.

This also makes some of that code look unsafe, like elementStyle->, but it's
probably fine.

lgtm

[rune, 2015-05-20 07:04:39]

The CQ bit was checked by rune@opera.com

[rune, 2015-05-20 07:04:39]

The patchset sent to the CQ was uploaded after l-g-t-m from esprehn@chromium.org
Link to the patchset: https://codereview.chromium.org/1133743004/#ps20001
(title: "Rebased")

[commit-bot: I haz the power, 2015-05-20 07:05:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1133743004/20001

[commit-bot: I haz the power, 2015-05-20 09:22:35]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=195606