This will let the front-end access them without pulling in the rest
of renderer_utils. The Serial class in particular will be useful for
capture/replay.

Also adds a very minimal unit test.

Bug: angleproject:4223
Change-Id: I9e63b8a8227a245b20a8f024b960fcf60c7840db
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1954611
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>