Do not observe MessageLoop destruction from ModelSafeWorker.

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 e752d0e670ec8f76c2a4684e969f2fc1eb9fde66..418882631e9e53d4c613c03393c506214f5debc9 100644
--- a/components/sync/test/engine/fake_model_worker.cc
+++ b/components/sync/test/engine/fake_model_worker.cc
@@ -4,10 +4,11 @@
 
 #include "components/sync/test/engine/fake_model_worker.h"
 
+#include "base/callback.h"
+
 namespace syncer {
 
-FakeModelWorker::FakeModelWorker(ModelSafeGroup group)
-    : ModelSafeWorker(nullptr), group_(group) {}
+FakeModelWorker::FakeModelWorker(ModelSafeGroup group) : group_(group) {}
 
 FakeModelWorker::~FakeModelWorker() {
   // We may need to relax this is FakeModelWorker is used in a
@@ -17,10 +18,6 @@ FakeModelWorker::~FakeModelWorker() {
   DCHECK(CalledOnValidThread());
 }
 
-void FakeModelWorker::RegisterForLoopDestruction() {
-  NOTREACHED();
-}
-
 SyncerError FakeModelWorker::DoWorkAndWaitUntilDoneImpl(
     const WorkCallback& work) {
   DCHECK(CalledOnValidThread());