GitLab

Introduce AXEventManager to maintain an ObserverList of objects
interested in views accessibility events.

This makes testing easier by eliminating the need for several
TestViewsDelegates.

BUG=908884
TEST=views_unittests, views_mus_unittests, browser_tests

Change-Id: Ic7017444fec7e694675ad675e0fefaf78b622130
Reviewed-on: https://chromium-review.googlesource.com/c/1352265Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612192}