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WindowedIncognitoObserver provides the interface for querying the
current status of incognito windows, and watch for new incognito window
opening. The interface can be used off the UI thread.

WindowedIncognitoMonitor is used to create WindowedIncognitoObserver
instances off the UI thread. Internally, the implementation class,
WindowedIncognitoMonitor, is registered as a BrowserListObserver
to observe opening/closing of incognito windows on behalf of queries
from WindowedIncognitoObserver.

Bug: 904785
Test: Add unit_tests WindowedIncognitoMonitorTest.*

Change-Id: I936d679c1963e1ffc1a5bbc5c4470274200d100c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1692208
Commit-Queue: Chinglin Yu <chinglinyu@chromium.org>
Reviewed-by: Gabriel Marin <gmx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683174}