Now that Array.from() always calls the runtime to set the length if it's
not equal to the current length, don't actually set it on the fast path
since it's unobservable and doesn't change anything.

Also remove check for the array being writable since it's no longer
needed.

Change-Id: I0928d80b445807912fd925f7957c9a76385fc6bc
Reviewed-on: https://chromium-review.googlesource.com/961403Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51929}