Removing the iterator from an array does not stop it being a fast array
(at least as far as BranchIfJSFastArray is concerned). The rest of the
code is not affected by whether the assert is false so this only
affected debug builds.

Change-Id: Iff78b8b21f3cc76a43d4251b4ba6ab078467db61
Reviewed-on: https://chromium-review.googlesource.com/976122Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52181}