Merely a minor refactor to clean up the diff of a subsequent CL. In the
follow up we will also distinguish the GL extents from the Vulkan
extents/layerCount pair.

Bug: angleproject:3189
Change-Id: Ibbb79bf2e6d3f798243b424bacbfb2e5538bfecf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1709755
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>