Instead of using the slow-path for COW arrays, we now properly copy
them and use the fast-path.

R=jgruber@chromium.org

Change-Id: Iebbad5f761d97c5400c457877571c7930269d52f
Reviewed-on: https://chromium-review.googlesource.com/1188130
Commit-Queue: Simon Zünd <szuend@google.com>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55384}