Reland "Add support for minimize/restore to AtkObjects"

Change to original: Modified implementation to use a WindowObserver
in the View accessibility code to reduce the test surface of
the original patch. This should eliminate issues with flaky tests.
Now instead of firing a "window minimization state changed" event
the code fires a "window visibility changed" event.

Original change's description:

> Add support for setting the AtkObject state and sending the
> appropriate AtkWindow events when a toplevel window in Chrome
> is minimized and restored.

Bug: 866336
Change-Id: I18636759730879762fcbe8e423a4537590241757
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1619700
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Martin Robinson <mrobinson@igalia.com>
Cr-Commit-Position: refs/heads/master@{#666223}
18 files changed