The WhichPowerOf2 function now takes arbitrary integer arguments, and
hence subsumes the WhichPowerOf2_64 function.

R=mstarzinger@chromium.org

Change-Id: I0692a17bbbe0074425d7c330342bc6fa34fb9d5c
Reviewed-on: https://chromium-review.googlesource.com/568495
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46646}