This class allows us to quickly notify Impl/Renderer layers of
which state changed.

BUG=angleproject:1040
TEST=angle_unittests (Mac/Win/Linux)

Change-Id: I2f27bc65b4f627a261e94e3e3b7095b169986cc6
Reviewed-on: https://chromium-review.googlesource.com/289557Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>