This is in preparation for using dynamic offsets for other sets than
the default descriptors.

Bug: angleproject:5736
Change-Id: I546e688ad57a35b380f8d7f303ffc0c62773ac18
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2804813
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>