This will provide a front-door entry point where we can get
a serialized string representing the context state. Previously
we would have to call ANGLE internal directly.

This will enable a refactor of the replays to work more like
standalone plugin libraries.

Bug: angleproject:5811
Change-Id: I5a97d7aa3cd70c49199264c2eb7ca953943ac3c3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2803391
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>