Let nacl builds use update.py instead of update.sh.
Let nacl builds use update.py instead of update.sh.

update.sh is gone in chromium, and using update.py will do the right
thing both before and after the deletion in chromium (previously,
update.py used to call update.sh internally on non-win).

No intended behavior change.

BUG=chromium:494442

Review URL: https://codereview.chromium.org/1496433003

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client