Take runtime path if startIndex parameter requires a ToInteger() call,
which can modify the elements kind of the receiver.

This removes a stub call from the builtin, and simplifies code slightly.

BUG=v8:5986
R=bmeurer@chromium.org, cbruni@chromium.org

Change-Id: Id238a81ab8ba28621858004b34d00a4356b8037f
Reviewed-on: https://chromium-review.googlesource.com/445006Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#43323}