Previously, the suggested file name(Always 'empty') returned by
'download_item->GetSuggestedFilename' is not the same with the default one saved
in local disk.
The patch reimplement this API allowing it to return the default file name, which
is more expected from user.
* Rename `completed` to `done`, making it align with Chromium's style.
* Add 'state' in `done` event's result. It can check the download item final
status: cancelled, completed, interrupted.
