This pattern never really took off. It was intended to be used to
unit-test our Validation logic, but it become so complex because
of interactions with Framebuffer::syncState that we could never
really make use of it.

Nuke it entirely and simplify the Context class.

Bug: angleproject:2372
Change-Id: I40b9d46ce7706511a210da496ee19192cf609366
Reviewed-on: https://chromium-review.googlesource.com/954291Reviewed-by: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>