This was uncovered by the Clang-win bot, which uses a different set of
warnings from MSVS.

BUG=angleproject:1327

Change-Id: Id298fee2df92752b77bf6e2b93307fa5311cf1fc
Reviewed-on: https://chromium-review.googlesource.com/332035Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>