This patch attempts to reduce the special handling of destructuring
assignments in arrow function parameters by "adopting" them from
wherever they were initially parsed into the arrow function's
FunctionState/Scope. This avoids incorrectly re-setting the
Scope of such assignments multiple times for arrow functions
that are nested inside other arrow params themselves.

It also generally seems better, in that we now only rewrite
destructuring assignments for a single function at a time.

Bug: chromium:807096
Change-Id: I6bef5613f99e3e8c130fc0aa2ee5d6fcf2efd34b
Reviewed-on: https://chromium-review.googlesource.com/900168Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Caitlin Potter <caitp@igalia.com>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51155}