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::StartConfiguration(Callback0::Type* callback) {
  core_thread_.message_loop()->PostTask(FROM_HERE, NewRunnableMethod(
    core_.get(),&SyncBackendHost::Core::DoStartConfiguration, callback));
}

void SyncBackendHost::StartConfiguration(Callback0::Type* callback) {
  core_thread_.message_loop()->PostTask(FROM_HERE, NewRunnableMethod(
    core_.get(),&SyncBackendHost::Core::DoStartConfiguration, callback));
}
