We assumed that every JSArray would have a JSObject as a prototype,
but it could be null, in which case we bail out to slow path.

Also rename spread_array variable here, because this fast-path
isn't just used by spreads anymore.

Bug: chromium:707675
Change-Id: I8045d83977735dd00c3ebde2e0704f6b04afdedd
Reviewed-on: https://chromium-review.googlesource.com/472907Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44531}