- Before getting the length property, we must check for it
  using [[GetOwnProperty]].  Also, if the obtained length
  is a number, we must properly convert it to an integer.

- In order to get the prototype we must use [[GetPrototypeOf]],
  and do so before checking the length.

R=cbruni@chromium.org, jkummerow@chromium.org
BUG=v8:1543
LOG=n

Review URL: https://codereview.chromium.org/1530893002

Cr-Commit-Position: refs/heads/master@{#32934}