[Sync] Make syncer commands avoid posting tasks on threads with no work to do

chrome/browser/sync/test/engine/fake_model_safe_worker_registrar.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/sync/test/engine/fake_model_safe_worker_registrar.h"
 
 #include "chrome/browser/sync/test/engine/fake_model_worker.h"
 
 namespace browser_sync {
 
 FakeModelSafeWorkerRegistrar::FakeModelSafeWorkerRegistrar(
     const ModelSafeRoutingInfo& routes) : routes_(routes) {
   std::set<ModelSafeGroup> groups;
   for (ModelSafeRoutingInfo::const_iterator it = routes_.begin();
        it != routes_.end(); ++it) {
     groups.insert(it->second);
   }
+  // Sessions always expect a passive worker to be present.
+  groups.insert(GROUP_PASSIVE);
 
   for (std::set<ModelSafeGroup>::const_iterator it = groups.begin();
        it != groups.end(); ++it) {
     workers_.push_back(make_scoped_refptr(new FakeModelWorker(*it)));
   }
 }
 
 FakeModelSafeWorkerRegistrar::~FakeModelSafeWorkerRegistrar() {}
 
 void FakeModelSafeWorkerRegistrar::GetWorkers(
     std::vector<ModelSafeWorker*>* out) {
   for (std::vector<scoped_refptr<ModelSafeWorker> >::const_iterator it =
            workers_.begin(); it != workers_.end(); ++it) {
     out->push_back(it->get());
   }
 }
 
 void FakeModelSafeWorkerRegistrar::GetModelSafeRoutingInfo(
     ModelSafeRoutingInfo* out) {
   *out = routes_;
 }
 
 }  // namespace browser_sync