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We track time with a GUI window in foreground and time with Crostini
running in the background. We do not yet track time with a terminal
browser window in foreground.
Note that the actual metric-emitting code is at
https://osscs.corp.google.com/chromium/chromium/src/+/master:components/guest_os/guest_os_engagement_metrics.cc;l=177;drc=4cc56e90bebaf1df23e738c19b88463cb4e2f366

Bug: chromium:1007139
Test: Deploy, use Crostini, check chrome://histograms
Change-Id: Ib7b0d6d88d7c0a4d94fcb3c399c41a243ccccbe1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2586649
Commit-Queue: David Munro <davidmunro@google.com>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Reviewed-by: Timothy Loh <timloh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838580}