These casts could result in overflow.

Bug: chromium:842028
Change-Id: I998b638c58333a29f6bc9136ae3e81b90683cb72
Reviewed-on: https://chromium-review.googlesource.com/1057415Reviewed-by: Luc Ferron <lucferron@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>