Revert of [Sync] Refactor ModelSafeWorker::DoWorkAndWaitUntilDone() to avoid code duplication.

components/sync/engine/passive_model_worker.cc

Index: components/sync/engine/passive_model_worker.cc
diff --git a/components/sync/engine/passive_model_worker.cc b/components/sync/engine/passive_model_worker.cc
index f8ba2d0392318872ab3c6d4699289116514dde94..6a4f76676e487ccf728959a044c67f298753e5be 100644
--- a/components/sync/engine/passive_model_worker.cc
+++ b/components/sync/engine/passive_model_worker.cc
@@ -4,7 +4,7 @@
 
 #include "components/sync/engine/passive_model_worker.h"
 
-#include <utility>
+#include "base/callback.h"
 
 namespace syncer {
 
@@ -12,8 +12,10 @@
 
 PassiveModelWorker::~PassiveModelWorker() {}
 
-void PassiveModelWorker::ScheduleWork(base::OnceClosure work) {
-  std::move(work).Run();
+SyncerError PassiveModelWorker::DoWorkAndWaitUntilDoneImpl(
+    const WorkCallback& work) {
+  // Simply do the work on the current thread.
+  return work.Run();
 }
 
 ModelSafeGroup PassiveModelWorker::GetModelSafeGroup() {