Add robust math to constant folding. Previously our multiplication and other operators could do overflows, which can lead to security bugs. BUG=chromium:637050 Change-Id: Icee22a87909e205b71bda1c5bc1627fcf5e26e90 Reviewed-on: https://chromium-review.googlesource.com/382678 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>