Bug 1579408 - Fix path manipulation in about support. r=Gijs

Use file.append() to create the desired path in a platform agnostic way.

Differential Revision: https://phabricator.services.mozilla.com/D45202

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

toolkit/content/aboutSupport.js

@@ -872,7 +872,8 @@ var snapshotFormatters = {
             "resource://gre/modules/kvstore.jsm"
           );
           let currProfDir = Services.dirsvc.get("ProfD", Ci.nsIFile);
-          let path = currProfDir.path + "/mediacapabilities";
+          currProfDir.append("mediacapabilities");
+          let path = currProfDir.path;
 
           function enumerateDatabase(name) {
             KeyValueService.getOrCreate(path, name)