Use a table for CSSSelectorParser's custom pseudo element use counter

This if cascade consumes both a fair bit of footprint while it makes the
switch slightly less readable.

Change-Id: Ia8cb0d14ec831ccfcf7f22154a12f16d44fbc9ad
Reviewed-on: https://chromium-review.googlesource.com/c/1329147
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607223}