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Track the session as open while there is at least one foreground window.
Log this in a dedicated app agent to avoid polluting AppState.
The same is done to IOSProfileSessionDurationsService's session
start/end events.

Bug: 1156586
Change-Id: I37586f989039206fbaa90f7ec4046fbf9582e70a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2599870
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Auto-Submit: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845653}