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ArcMetricsService used to be simple, but it's not anymore and needs
some tests. For example, since b/62703685 has been fully implemented,
ArcMetricsService::ReportBootProgress() may receive event timestamps
that are smaller than |arc_start_time|.

BUG=b:65128488
BUG=b:62703685
TEST=try

Change-Id: Idd2aed6bdc2cbdb970d8e9b0e02987377ae1cfd6
Reviewed-on: https://chromium-review.googlesource.com/785110Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519739}