Fixes tests related to integer attributes in
dEQP-GLES3.functional.shaders.builtin_functions.*

BUG=angleproject:880

Change-Id: I107c5c9fd1adc29f58617fab095374daa74a0da1
Reviewed-on: https://chromium-review.googlesource.com/298970Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>