It's still only valid to bind a 2D texture but the validation layer
verifies that.

BUG=angle:681

Change-Id: I744dc32f9bd0f69b1e1235b8feef7e796167d033
Reviewed-on: https://chromium-review.googlesource.com/236931Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>