Ensure AX notification waiter listens to inner frame trees

As part of fixing bug 1454778 and exploring how to deflake UIA tests,
we're going to start enforcing that the AX tree is clean at the end of
all dump tests. As soon as we enforce this, the fenced frame dump test
will start failing because the notification waiter never receives
notifications from fenced frames, so the test times out. This was the
core issue that CL 4503308 was trying to solve, and is likely the cause of flakiness behind issue 1459385.

To prepare for enforcing clean AX trees at the end of dump tests, we'll make sure the notification waiter can listen to inner frame trees and remove the hacky timeout in the test.

Bug: 1454778,1428967,1459385
Change-Id: I96ec0c7374a6de1e498f28a1ed8616cea96d45ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4777428


Commit-Queue: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Reviewed-by: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1184833}