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::RemoveParentJsEventRouter() {
  DCHECK_EQ(MessageLoop::current(), host_->frontend_loop_);
  parent_router_ = NULL;
  MessageLoop* core_message_loop = host_->core_thread_.message_loop();
  CHECK(core_message_loop);
  core_message_loop->PostTask(
      FROM_HERE,
      NewRunnableMethod(this,
                        &SyncBackendHost::Core::DisconnectChildJsEventRouter));
}

void SyncBackendHost::Core::RemoveParentJsEventRouter() {
  DCHECK_EQ(MessageLoop::current(), host_->frontend_loop_);
  parent_router_ = NULL;
  MessageLoop* core_message_loop = host_->core_thread_.message_loop();
  CHECK(core_message_loop);
  core_message_loop->PostTask(
      FROM_HERE,
      NewRunnableMethod(this,
                        &SyncBackendHost::Core::DisconnectChildJsEventRouter));
}
