They are only allowed in the glVertexAttribPointer call.

BUG=angleproject:1101

Change-Id: I85aae9e35b45e8b1296244756091271d01dd1533
Reviewed-on: https://chromium-review.googlesource.com/376719Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>