Request a prompt interface for the given prompt reason;
@throws NS_ERROR_NOT_AVAILABLE if no prompt is allowed or
available for the given reason.
| aPromptReason | The reason for the auth prompt; one of #PROMPT_NORMAL or #PROMPT_PROXY |
| iid | The desired interface, e.g. NS_GET_IID(nsIAuthPrompt2). |
| an nsIAuthPrompt2 interface, or throws NS_ERROR_NOT_AVAILABLE |
Normal (non-proxy) prompt request.
Proxy auth request.