The zlib roll 50a8d0e0 broke the Window/gyp/ninja build.
Suppress C4245.

BUG=angleproject:2004

Change-Id: Ifdac7a1ec3162c8a296f6d4919a2b6f83bdf1e50
Reviewed-on: https://chromium-review.googlesource.com/483802Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>