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

components/sync/test/engine/fake_model_worker.cc

Index: components/sync/test/engine/fake_model_worker.cc
diff --git a/components/sync/test/engine/fake_model_worker.cc b/components/sync/test/engine/fake_model_worker.cc
index c696005097602c6f52fb85739328138701729aca..06dac92ad0fce35f918b1fbcc5a028bdf1720275 100644
--- a/components/sync/test/engine/fake_model_worker.cc
+++ b/components/sync/test/engine/fake_model_worker.cc
@@ -4,8 +4,6 @@
 
 #include "components/sync/test/engine/fake_model_worker.h"
 
-#include "base/callback.h"
-
 namespace syncer {
 
 FakeModelWorker::FakeModelWorker(ModelSafeGroup group) : group_(group) {}
@@ -18,11 +16,10 @@ FakeModelWorker::~FakeModelWorker() {
   DCHECK(thread_checker_.CalledOnValidThread());
 }
 
-SyncerError FakeModelWorker::DoWorkAndWaitUntilDoneImpl(
-    const WorkCallback& work) {
+void FakeModelWorker::ScheduleWork(base::Closure work) {
   DCHECK(thread_checker_.CalledOnValidThread());
   // Simply do the work on the current thread.
-  return work.Run();
+  work.Run();
 }
 
 ModelSafeGroup FakeModelWorker::GetModelSafeGroup() {