Fix crash in CreateAuthenticatorFactory().

CreateAuthenticatorFactory() is called asynchronously, but it didn't handle
the case when it's called after host object is destroyed.

BUG=150644

Review URL: https://codereview.chromium.org/11090036

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@161077 0039d316-1c4b-4281-b951-d872f2087c98
  void SigTermHandler(int signal_number) {
    DCHECK(signal_number == SIGTERM);
    DCHECK(context_->network_task_runner()->BelongsToCurrentThread());
    LOG(INFO) << "Caught SIGTERM: Shutting down...";
    Shutdown(kSuccessExitCode);
  }

  void SigTermHandler(int signal_number) {
    DCHECK(signal_number == SIGTERM);
    DCHECK(context_->network_task_runner()->BelongsToCurrentThread());
    LOG(INFO) << "Caught SIGTERM: Shutting down...";
    Shutdown(kSuccessExitCode);
  }
