That way we can properly walk the active ambiguous stack of expressions and
stop where it's non-ambiguous. In the bug we would have forced context
allocation of "this" in an outer function because an inner function was parsed
as part of an arrow function head and "this" was referenced. That caused the
ambiguous arrow head scope to be marked, even though the reference came from a
non-ambiguous function.

Bug: chromium:930580
Change-Id: I0bf0fa569e2d2ca1dc26b0514fe5bdb48ab7ae6f
Reviewed-on: https://chromium-review.googlesource.com/c/1462005Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59488}