These replace the ::create methods, to facilitate constexpr versions.
Also adds a constructor of WorkGroupSize that can be made to be
constexpr.

Bug: angleproject:1432
Change-Id: I413d1a56102c94d6a383775110a2b850a7532ea0
Reviewed-on: https://chromium-review.googlesource.com/776279
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>