And merge that with TheNUllBuffer as well so that you only have one
dummy buffer per context.

Bug: b/161391337
Change-Id: I75fddb5c48393876e745ff237e11d9c5672ae10e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2300707
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>