Bug 1539265 - Add ::marker to the list of suggested pseudo selectors. r=jdescottes

For autocompletion of : in selector contexts, like the Style Editor.

Differential Revision: https://phabricator.services.mozilla.com/D25110

--HG--
extra : moz-landing-system : lando

devtools/server/actors/inspector/walker.js

@@ -88,6 +88,7 @@ const PSEUDO_SELECTORS = [
   [":disabled", 0],
   [":checked", 1],
   ["::selection", 0],
+  ["::marker", 0],
 ];
 
 const HELPER_SHEET = "data:text/css;charset=utf-8," + encodeURIComponent(`