In preparation for compute pipeline and dirty bits.

Bug: angleproject:3562
Change-Id: I611b97fee1e500fdad32355d096254fd787c4ddd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1703525Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>