Tells the content handler to take over handling the content. If this
function succeeds, the URI Loader will leave this request alone, ignoring
progress notifications. Failure of this method will cause the request to be
cancelled, unless the error code is NS_ERROR_WONT_HANDLE_CONTENT (see
below).
@throw NS_ERROR_WONT_HANDLE_CONTENT Indicates that this handler does not
want to handle this content. A different way for handling this
content should be tried.
| aWindowContext | Window context, used to get things like the current nsIDOMWindow for this request. May be null. |
| aContentType | The content type of aRequest |
| aRequest | A request whose content type is already known. |