[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 6a4f76676e487ccf728959a044c67f298753e5be..3204fbfa4d44a124f0670b5a1db3f79bd015384c 100644
--- a/components/sync/engine/passive_model_worker.cc
+++ b/components/sync/engine/passive_model_worker.cc
@@ -4,18 +4,14 @@
 
 #include "components/sync/engine/passive_model_worker.h"
 
-#include "base/callback.h"
-
 namespace syncer {
 
 PassiveModelWorker::PassiveModelWorker() = default;
 
 PassiveModelWorker::~PassiveModelWorker() {}
 
-SyncerError PassiveModelWorker::DoWorkAndWaitUntilDoneImpl(
-    const WorkCallback& work) {
-  // Simply do the work on the current thread.
-  return work.Run();
+void PassiveModelWorker::ScheduleWork(base::Closure work) {
+  work.Run();
 }
 
 ModelSafeGroup PassiveModelWorker::GetModelSafeGroup() {