[SyncFS] Clean up SequencedWorkerPool in SyncEngineTest

BUG=None

Review URL: https://codereview.chromium.org/595273003

Cr-Commit-Position: refs/heads/master@{#296376}

chrome/browser/sync_file_system/drive_backend/sync_engine_unittest.cc

@@ -73,6 +73,10 @@ class SyncEngineTest : public testing::Test,
     sync_engine_.reset();
     WaitForWorkerTaskRunner();
     worker_pool_->Shutdown();
+
+    worker_task_runner_ = NULL;
+    worker_pool_ = NULL;
+
     base::RunLoop().RunUntilIdle();
   }