Fix a few instances of MSVC warning C4244 (possible value truncation).

BUG=81439
TEST=none

Review URL: https://codereview.chromium.org/776823002

Cr-Original-Commit-Position: refs/heads/master@{#306652}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: a36236427f080e5e8cf28e1390452addf48b1714