This adds a convenience method for the common Smi to int conversion
pattern.

Bug: 
Change-Id: I7d7b171c36cfec5f6d10c60f1d9c3e06e3aed0fa
Reviewed-on: https://chromium-review.googlesource.com/563205
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Andreas Rossberg <rossberg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46516}