MD Settings: fix metrics reporting unittests on official testers -- take 2

R=dschuyler@chromium.org
BUG=637068

Review-Url: https://codereview.chromium.org/2265553003
Cr-Commit-Position: refs/heads/master@{#414270}

chrome/browser/ui/webui/settings/metrics_reporting_handler_unittest.cc

@@ -61,7 +61,12 @@ class MetricsReportingHandlerTest : public testing::Test {
   }
 
   void TearDown() override {
+    // For crbug.com/637068 which only run on official bots with no try jobs.
+    base::RunLoop().RunUntilIdle();
     handler_.reset();
+    base::RunLoop().RunUntilIdle();
+    local_state_.reset();
+    base::RunLoop().RunUntilIdle();
   }
 
   PrefService* local_state() { return local_state_->Get(); }