Fix compile error on 32-bit builds.

BUG=angle:520

Change-Id: Ibd3f46ca565ddd96820ef52b140f7e99cf28a5c2
Reviewed-on: https://chromium-review.googlesource.com/223168
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>