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
  bool OnHostDomainPolicyUpdate(const std::string& host_domain) {
    DCHECK(context_->network_task_runner()->BelongsToCurrentThread());

    if (!host_domain.empty() &&
        !EndsWith(xmpp_login_, std::string("@") + host_domain, false)) {
      Shutdown(kInvalidHostDomainExitCode);
    }
    return false;
  }

  bool OnHostDomainPolicyUpdate(const std::string& host_domain) {
    DCHECK(context_->network_task_runner()->BelongsToCurrentThread());

    if (!host_domain.empty() &&
        !EndsWith(xmpp_login_, std::string("@") + host_domain, false)) {
      Shutdown(kInvalidHostDomainExitCode);
    }
    return false;
  }
