Non-capturing groups so far did not have any effect, so we elided
them in the regexp parser. However, they do affect how lookarounds
are parsed: in unicode mode, lookarounds are not quantifiable. They
are however if wrapped in a group.

BUG=v8:5845

Review-Url: https://codereview.chromium.org/2636883002
Cr-Commit-Position: refs/heads/master@{#42436}