GitLab

Created AXEventCounter class to help tests counting accessibility
events. Instead of writing their own observers, tests can create an
instance of AXEventCounter and check how many events of each type
have been registered.

Bug: None
Change-Id: I3b07a077a680b98415f5ed6bee1c4e6528237a22
AX-Relnotes: no user-facing changes.
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2336603
Commit-Queue: Jacobo Aragunde Pérez <jaragunde@igalia.com>
Reviewed-by: Peter Boström <pbos@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800989}