WebKit uses the -Wshorten-64-to-32 flag which warns on these cases.

Bug: 3439
Change-Id: I8c1de60da0f173ca2036e2120e79b857f5f2775f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1740866
Commit-Queue: James Darpinian <jdarpinian@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>