js: use System.gc() instead of shell_global_gc()

gjs now offers a gc() method in the System module, no need to use our
own.

https://bugzilla.gnome.org/show_bug.cgi?id=679832

js/perf/core.js

@@ -1,5 +1,7 @@
 // -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
 
+const System = imports.system;
+
 const Main = imports.ui.main;
 const Scripting = imports.ui.scripting;
 
@@ -99,7 +101,7 @@ function run() {
         Main.overview.hide();
         yield Scripting.waitLeisure();
 
-        global.gc();
+        System.gc();
         yield Scripting.sleep(1000);
         Scripting.collectStatistics();
         Scripting.scriptEvent('afterShowHide');