Fix flakiness in EventBasedStatusReportingServiceTest

by shutting down SystemClockClient as appropriate.

Bug: 939160
Change-Id: I145b2b96e602b412340e56858172a66e9dd8dca7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1507216Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638402}

chrome/browser/chromeos/child_accounts/event_based_status_reporting_service_unittest.cc

@@ -124,6 +124,7 @@ class EventBasedStatusReportingServiceTest : public testing::Test {
     service_->Shutdown();
     arc_test_.TearDown();
     profile_.reset();
+    SystemClockClient::Shutdown();
     PowerManagerClient::Shutdown();
   }