Pass a new |popup_id| parameter to OnBeforePopup and call a new
OnBeforePopupAborted callback if the popup is aborted before
OnAfterCreated is called for the popup browser. Add new
CefBrowserHost::GetBrowserByIdentifier and GetOpenerIdentifier
methods to assist with retrieval of associated browsers.
In cefclient, clean up state when a popup is aborted and close
any associated popup browsers when the opener browser is closed.
This also works when running with `--use-default-popup`.
