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This change implements ArcMetricsService::ReportAppKill, which is
called by the guest to report lmkd and OOM kills. These are passed
on to MemoryKillsMonitor, which manages the corresponding UMA metrics.

BUG=b:165194376
TEST=atest lmkd_unit_test; monitor Chrome log file to verify that
 MemoryKillsMonitor has received kill events.

Change-Id: I55f1800856388e248a29de0478bb983d3f87a4ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463770
Commit-Queue: Willie Koomson <wvk@google.com>
Reviewed-by: Josh Horwich <jhorwich@chromium.org>
Reviewed-by: Cheng-Yu Lee <cylee@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819119}