gets introduced

This tracks barriers in an array based on dstPipelineStage.

Bug: b/155341891
Change-Id: Icba2ef81530edcdb9ae363b00f0e7b9efe93d48f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2188955Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Charlie Lao <cclao@google.com>