This CL fixes a bug if the second argument ('from') for lastIndexOf
changes the array when its converted to an integer.

R=jgruber@chromium.org

Bug: chromium:878845
Change-Id: I8759dd19381c63f0dde1d4c5abc1b6c7291c6048
Reviewed-on: https://chromium-review.googlesource.com/1196507Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Simon Zünd <szuend@google.com>
Cr-Commit-Position: refs/heads/master@{#55525}