Enable HistoryModelWorker by default, now that bug 69561 is fixed.

BUG=69561
TEST=Run sync manually and run integration tests, sync should not crash.

Review URL: http://codereview.chromium.org/7016007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@85211 0039d316-1c4b-4281-b951-d872f2087c98
void SyncBackendHost::Core::ConnectChildJsEventRouter() {
  DCHECK_EQ(MessageLoop::current(), host_->core_thread_.message_loop());
  if (!syncapi_->GetJsBackend()->GetParentJsEventRouter()) {
    syncapi_->GetJsBackend()->SetParentJsEventRouter(this);
    syncapi_->AddObserver(&sync_manager_observer_);
  }
}

void SyncBackendHost::Core::ConnectChildJsEventRouter() {
  DCHECK_EQ(MessageLoop::current(), host_->core_thread_.message_loop());
  if (!syncapi_->GetJsBackend()->GetParentJsEventRouter()) {
    syncapi_->GetJsBackend()->SetParentJsEventRouter(this);
    syncapi_->AddObserver(&sync_manager_observer_);
  }
}
