Use FlushForTesting in cros_healthd metric provider test

Use new method ServiceConnection::FlushForTesting instead of
RunUntilIdle.

Change-Id: I032a428465ac661905ac3cfe466a898c4ca015bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2398722Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Oleh Lamzin <lamzin@google.com>
Cr-Commit-Position: refs/heads/master@{#805655}

chrome/browser/metrics/cros_healthd_metrics_provider_unittest.cc

@@ -71,9 +71,8 @@ class CrosHealthdMetricsProviderTest : public testing::Test {
   ~CrosHealthdMetricsProviderTest() override {
     chromeos::CrosHealthdClient::Shutdown();
 
-    // Wait for cros_healthd::ServiceConnection to observe the destruction of
-    // the client.
-    base::RunLoop().RunUntilIdle();
+    // Wait for ServiceConnection to observe the destruction of the client.
+    chromeos::cros_healthd::ServiceConnection::GetInstance()->FlushForTesting();
   }
 
  protected: