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As of 10.15, the system seems to try to activate every app shortly
after the app starts up. This causes windows the app creates early
in its lifecycle to be given key status by appkit, even if they were
created without being given key status. In order to avoid that, tests
that depend on this behavior need to wait for the system activation
to happen before creating their windows. This change adds a mechanism
for those tests to do so, and adds appropriate uses of it.

Bug: 988868
Change-Id: I219c926a8f716ad938e0f5279c04b9f94dba1c24
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1853107Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705600}