It's really only used to track the render pass command buffer.  The
change is made in preparation for compute, where the dispatcher command
buffer can change without affecting the render pass.

Bug: angleproject:3562
Change-Id: Ia8246de731d5c6a272fa17d6f8952dc981eca36e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1703523Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>