Before, we were treating objects with the builtin ArrayValues iterator
method as array-like, where the iterator would iterate through to the
full length of the object.

This optimization was not sound, because it does not ensure that the
next method hasn't been modified. Even if it hasn't been modified,
it's entirely possible to be modified during iteration. Thus, this
optimization has been removed due to its observability.

BUG=v8:5699
R=littledan@chromium.org, cbruni@chromium.org

Review-Url: https://codereview.chromium.org/2544503002
Cr-Commit-Position: refs/heads/master@{#41394}