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Adds a VoiceOver notification when the number of visible Chrome windows
changes, if multiple windows are possible.

This is complicated by some window manipulations causing transient
changes in the number of foregrounded windows -- for example, replacing
one window with another will briefly have three scenes with Foreground-
Inactive states.

To mitigate this, the app agent delays notifications, and coalesces
multiple notifications in the delay period into a single notification.

(No notification is posted if the window count doesn't change from the
last notification).

Bug: 1137818
Change-Id: I2a308072c92aceb3816397dae6fdf50d61d14398
Skip-Translation-Screenshots-Check: True
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2540670
Commit-Queue: Mark Cogan <marq@chromium.org>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/master@{#828420}