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This is a tricky test due to the asynchronous nature of the event. It
is produced in response to a system window event, so we need the test
to produce an actual window so we can listen to it, and we need to add
a way to wait for it.

Bug: None
Change-Id: If7cd10c5db282f7cc465505286790ab1f4b458a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2370984
Commit-Queue: Jacobo Aragunde Pérez <jaragunde@igalia.com>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804550}