This concept isn't strictly necessary for GL-side validation. Instead
we can use a bitset to track active attribs, and determine is a
particular location is active.

BUG=angleproject:1123

Change-Id: If7a920a3071672116bafffb3368671f721723b65
Reviewed-on: https://chromium-review.googlesource.com/294570Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>