This is essentially the same commit as fed0dda1, but with the BaseVertex variant.

Bug: angleproject:4373
Change-Id: I2d6fe46a0e8ce5faa8c92d4db6909f8cabd9692f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2032972Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>