And fix RoundUpToPowerOfTwo32 to return 1 for the input 0.
0 is no power of two.
Beside being the correct value, this also avoids a special case in the
(new) fast path using the number of leading zeros.

R=jochen@chromium.org, ahaas@chromium.org

Change-Id: I87173495e13b334954bcebbb55724fb666dfa809
Reviewed-on: https://chromium-review.googlesource.com/488143Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44925}