[gnome-shell/wip/carlosg/osk-emoji-and-keypad: 16/22] keyboard: Fix JS warning

[Carlos Garnacho <carlosg src gnome org>]

commit 37e96df3722b5c0bcb6781fc371dde3572990578
Author: Carlos Garnacho <carlosg gnome org>
Date:   Mon Jan 21 21:40:10 2019 +0100

    keyboard: Fix JS warning
    
    The solution is pointed out by the warning itself.

 js/ui/keyboard.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/js/ui/keyboard.js b/js/ui/keyboard.js
index e293ee7ea..c58ab81e4 100644
--- a/js/ui/keyboard.js
+++ b/js/ui/keyboard.js
@@ -288,7 +288,7 @@ var Key = new Lang.Class({
     },
 
     _getKeyval(key) {
-        let unicode = String.charCodeAt(key, 0);
+        let unicode = String.prototype.charCodeAt(key, 0);
         return Gdk.unicode_to_keyval(unicode);
     },