Fixes conformance/reading/read-pixels-pack-alignment.html

BUG=angleproject:885

Change-Id: I48f1598d9a67f3980d2ff86307eb107ec7506776
Reviewed-on: https://chromium-review.googlesource.com/275691Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>