[Sync] Move thread checking into the ModelSafeWorker interface.

components/sync/driver/glue/sync_backend_host_impl.cc

Index: components/sync/driver/glue/sync_backend_host_impl.cc
diff --git a/components/sync/driver/glue/sync_backend_host_impl.cc b/components/sync/driver/glue/sync_backend_host_impl.cc
index 2b339724cd9b06a23be89d9788f37a0640a810dc..06d6c1ea71f9153aeffe00abb448733b702eac0d 100644
--- a/components/sync/driver/glue/sync_backend_host_impl.cc
+++ b/components/sync/driver/glue/sync_backend_host_impl.cc
@@ -73,8 +73,6 @@ SyncBackendHostImpl::~SyncBackendHostImpl() {
 void SyncBackendHostImpl::Initialize(
     SyncFrontend* frontend,
     base::Thread* sync_thread,
-    const scoped_refptr<base::SingleThreadTaskRunner>& db_thread,
-    const scoped_refptr<base::SingleThreadTaskRunner>& file_thread,
     const WeakHandle<JsEventHandler>& event_handler,
     const GURL& sync_service_url,
     const std::string& sync_user_agent,
@@ -90,8 +88,7 @@ void SyncBackendHostImpl::Initialize(
   CHECK(sync_thread);
   sync_thread_ = sync_thread;
 
-  registrar_ = base::MakeUnique<SyncBackendRegistrar>(
-      name_, sync_client_, ui_thread_, db_thread, file_thread);
+  registrar_ = base::MakeUnique<SyncBackendRegistrar>(name_, sync_client_);
 
   DCHECK(frontend);
   frontend_ = frontend;