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During shutdown of JankMonitor, when the monitor timer is fired after
MetricSource has shut down, the timer dereferences the null pointer and
crashes. Fix the crash by nulling out the pointers after the monitor
timer shut down.

Bug: 1015425
Test: JankMonitorShutdownTest.ShutdownRace_TimerFired
Change-Id: Icd9073e4527e0ac01c373fe486d88aa0985a5841
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1868772
Commit-Queue: Chinglin Yu <chinglinyu@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709822}