Updated AccessibilityNotificationWaiter to also wait for focus events

AccessibilityNotificationWaiter is part of the accessibility browser tests infrastructure.
Focus events are fired via a different mechanism in BrowserAccessibilityManager than all other Blink-based or generated events.
In order to write effective tests that check if a focus event has been fired,
we need to modify AccessibilityNotificationWaiter to watch for focus events too.

Change-Id: I1e8730442cd733f51b7510592e0f9f0bc6f3c2a3
Bug: 982776
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1692104
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676140}
7 files changed