Make SyncEngineTest's SequencedWorkerPool multi-threaded.

BUG=646443

Review-Url: https://codereview.chromium.org/2339253004
Cr-Commit-Position: refs/heads/master@{#419149}

chrome/browser/sync_file_system/drive_backend/sync_engine_unittest.cc

@@ -32,7 +32,7 @@ class SyncEngineTest : public testing::Test,
  public:
   typedef RemoteFileSyncService::OriginStatusMap RemoteOriginStatusMap;
 
-  SyncEngineTest() : worker_pool_owner_(1, "Worker") {}
+  SyncEngineTest() : worker_pool_owner_(2, "Worker") {}
   ~SyncEngineTest() override {}
 
   void SetUp() override {