patch 8.1.1909: more functions can be used as methods

Problem:    More functions can be used as methods.
Solution:   Make a few more functions usable as a method.
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index f742035..23041d1 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -2332,8 +2332,7 @@
 endfunc
 
 func Test_popupwin_recycle_bnr()
-  let winid = popup_notification('nothing wrong', {})
-  let bufnr = winbufnr(winid)
+  let bufnr = popup_notification('nothing wrong', {})->winbufnr()
   call popup_clear()
   let winid = popup_notification('nothing wrong', {})
   call assert_equal(bufnr, winbufnr(winid))