Also add a test in StateChangeTest.cpp to validate the behavior.

Bug: angleproject:2458

Change-Id: I58848772c0b4f71aaa3ee187778e49fa08e6800d
Reviewed-on: https://chromium-review.googlesource.com/1007320Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>
Commit-Queue: Luc Ferron <lucferron@chromium.org>