Do not take the fast path for Array.prototype.keys() if the array length is not
guaranteed to be an Smi.

BUG=660925, v8:5388
R=bmeurer@chromium.org, mstarzinger@chromium.org

Review-Url: https://codereview.chromium.org/2496323002
Cr-Commit-Position: refs/heads/master@{#40976}