This will allow the capture/replay tool to easily intercept and label
attribute locations for remapping.

There's some inconsistency in implementation in the GL desktop front-
end. This is a quick fix and the full implementation is left for when
we implement the full desktop GL API set.

Bug: angleproject:4598
Change-Id: Ibf11bcb8669d27265ea376494a2e3124825cf3be
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2171933
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>