This would start off the zeroth element as enabled even if there was
no attachment. If we remove the extra set, then we no longer have to
check if the attachment is valid when iterating over the enabled
bitset.

BUG=angleproject:2107

Change-Id: I679f66156ced09d42add9e302ac974166713e0d5
Reviewed-on: https://chromium-review.googlesource.com/581884Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>