Remove broken sibling selector optimization

Remove broken sibling selector optimization

Blink r154053 landed with a broken optimization for nth-child that
this patch removes.

Background: nth-child was formerly O(1) and nth-last-child was O(N^2),
but r154053 reversed these making the more common nth-child slower.
It included an optimization to make both O(1), but the optimization
was broken in one of the merges.

I will re-land the working optimization in another patch, and with
a magnitude perf test Ojan, eseidel, and myself wrote to ensure it
works.

R=ojan@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=154072

[ojan, 2013-07-12 01:19:30]

lgtm

[commit-bot: I haz the power, 2013-07-12 01:19:56]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/leviw@chromium.org/18147020/1

[eseidel, 2013-07-12 03:50:24]

Committed patchset #1 manually as r154072 (presubmit successful).