The ES6 spec for this function declares that ToLength
should only be called once. We were evaluating it multiple
times, so if length was an object with a valueOf method,
we could see effects take place multiple times.

R=littledan@chromium.org
LOG=N
BUG=v8:4218

Review URL: https://codereview.chromium.org/1237583005

Cr-Commit-Position: refs/heads/master@{#30240}