Previously the array iterator protector only guarded the lookup of the
@@iterator symbol on the initial Array.prototype, and we had to use an
additional map check on the %ArrayIteratorPrototype% to ensure that no
one messed with the next() method.  This CL extends the array iterator
protector to also guard the lookup of %ArrayIteratorPrototype%.next.

This simplifies the code quite a bit and makes it more robust for cases
where someone has to install additional methods on the iterator
prototype, i.e. a custom async iterator.

Bug: v8:7510, v8:7514
Change-Id: Ie6080bb837a91a2b60b224597121470614210660
Reviewed-on: https://chromium-review.googlesource.com/945728Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51684}