Fix flakiness in CrasAudioClientTest

Bug: none
Change-Id: If821edbfc5ea2f222c0a885cbe14d8a48c789d0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1575868
Auto-Submit: Evan Stade <estade@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652867}

chromeos/dbus/audio/cras_audio_client_unittest.cc

@@ -365,7 +365,10 @@ class CrasAudioClientTest : public testing::Test {
     base::RunLoop().RunUntilIdle();
   }
 
-  void TearDown() override { mock_bus_->ShutdownAndBlock(); }
+  void TearDown() override {
+    mock_bus_->ShutdownAndBlock();
+    CrasAudioClient::Shutdown();
+  }
 
  protected:
   // A callback to intercept and check the method call arguments.