[Sync] Move thread checking into the ModelSafeWorker interface.

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 623b31961526b5d5ec0bc078dc1d64c324bdbfd2..6a4f76676e487ccf728959a044c67f298753e5be 100644
--- a/components/sync/engine/passive_model_worker.cc
+++ b/components/sync/engine/passive_model_worker.cc
@@ -22,4 +22,10 @@ ModelSafeGroup PassiveModelWorker::GetModelSafeGroup() {
   return GROUP_PASSIVE;
 }
 
+bool PassiveModelWorker::IsOnModelThread() {
+  // Passive types are checked by SyncBackendRegistrar.
+  NOTREACHED();
+  return false;
+}
+
 }  // namespace syncer