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).

This also has the benefit of working on Windows.

No intended behavior change.

BUG=chromium:494442
LOG=n

Review URL: https://codereview.chromium.org/1495653002

Cr-Commit-Position: refs/heads/master@{#32529}