This completes the basic refactor to the Observer pattern.

Bug: angleproject:2372
Change-Id: I810deff7c7e39baa64b57ce2a79cd732b1af7c34
Reviewed-on: https://chromium-review.googlesource.com/940862
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>