[SyncFS] Clean up SequencedWorkerPool in SyncEngineTest

chrome/browser/sync_file_system/drive_backend/sync_engine_unittest.cc

Index: chrome/browser/sync_file_system/drive_backend/sync_engine_unittest.cc
diff --git a/chrome/browser/sync_file_system/drive_backend/sync_engine_unittest.cc b/chrome/browser/sync_file_system/drive_backend/sync_engine_unittest.cc
index 65b2cc52a6ee73b24fdfe3a4c2bf9a86af7c5347..c680bab01d23a1ada6b853bd25003abf82f8e3f1 100644
--- a/chrome/browser/sync_file_system/drive_backend/sync_engine_unittest.cc
+++ b/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();
   }