[polari] roomList: Clarify a helper function

[Florian Müllner <fmuellner src gnome org>]

commit fafe095e4b0ccf3e2d19134313a87222901ecccc
Author: Florian Müllner <fmuellner gnome org>
Date:   Thu Apr 4 00:31:23 2019 +0200

    roomList: Clarify a helper function
    
    for-loops with multiple variables are hard to read, come up with
    an easier (and more concise) alternative.
    
    https://gitlab.gnome.org/GNOME/polari/merge_requests/109

 src/roomList.js | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)
---
diff --git a/src/roomList.js b/src/roomList.js
index ca10a5a..a033fab 100644
--- a/src/roomList.js
+++ b/src/roomList.js
@@ -524,12 +524,8 @@ class RoomList extends Gtk.ListBox {
     }
 
     _roomToRowIndex(index) {
-        let nChildren = this.get_children().length;
-        for (let i = 0, roomIndex = 0; i < nChildren; i++) {
-            if (this.get_row_at_index(i).room && roomIndex++ == index)
-                return i;
-        }
-        return -1;
+        let roomRows = this.get_children().filter(r => r instanceof RoomRow);
+        return index < roomRows.length ? roomRows[index].get_index() : -1;
     }
 
     _getRoomRowAtIndex(index) {