Previously, Strings without an iterator would go to the runtime path
and fail on because it expected a JSReceiver type. This was in-line
with what the elements accessor expected. We can actually handle all
object types in the final slow path (using LookupIterator) so it is no
problem to change the accept types.

Bug: chromium:816289
Change-Id: Iebb8de0bb7551aee3894c8a23836d079c93726a7
Reviewed-on: https://chromium-review.googlesource.com/937461Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51574}