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 OnNatPolicyUpdate(bool nat_traversal_enabled) {
    DCHECK(context_->network_task_runner()->BelongsToCurrentThread());

    if (allow_nat_traversal_ != nat_traversal_enabled) {
      allow_nat_traversal_ = nat_traversal_enabled;
      LOG(INFO) << "Updated NAT policy.";
      return true;
    }
    return false;
  }

  bool OnNatPolicyUpdate(bool nat_traversal_enabled) {
    DCHECK(context_->network_task_runner()->BelongsToCurrentThread());

    if (allow_nat_traversal_ != nat_traversal_enabled) {
      allow_nat_traversal_ = nat_traversal_enabled;
      LOG(INFO) << "Updated NAT policy.";
      return true;
    }
    return false;
  }
