Fix warnings from size_t conversions. These casts could result in overflow. Bug: chromium:842028 Change-Id: I998b638c58333a29f6bc9136ae3e81b90683cb72 Reviewed-on: https://chromium-review.googlesource.com/1057415 Reviewed-by: Luc Ferron <lucferron@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>