Instead of using defines in the header, use the same approach as we do
with frame capture by defining a stub "mock" cpp file.

Bug: angleproject:5805
Change-Id: Ief1cb6497ddafc9656bb0e7d6a921eff3610a7fb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2801695Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>