Until now lookbehind assertions have been quantifiable in non-unicode regexps.
This seems to be an oversight in the spec.

R=jgruber@chromium.org

Bug: v8:7462
Change-Id: Iad0db441089c7510dd2c42a861db92c05545ce1e
Reviewed-on: https://chromium-review.googlesource.com/926102
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51392}