Modify some more GC layout tests to work with Ignition

third_party/WebKit/LayoutTests/netinfo/gc-unused-listeners.html

Index: third_party/WebKit/LayoutTests/netinfo/gc-unused-listeners.html
diff --git a/third_party/WebKit/LayoutTests/netinfo/gc-unused-listeners.html b/third_party/WebKit/LayoutTests/netinfo/gc-unused-listeners.html
index e5cda4e75837597004c0c8abd73918e8b30d993c..6ff207c83cbfa136ea5b450a3b284c4de8d2901a 100644
--- a/third_party/WebKit/LayoutTests/netinfo/gc-unused-listeners.html
+++ b/third_party/WebKit/LayoutTests/netinfo/gc-unused-listeners.html
@@ -11,7 +11,7 @@ description('Tests that unused listeners are collected.');
 shouldBe('typeof window.internals.observeGC', '"function"',
 'this test requires window.internals');
 
-var callback = function(e) {
+let callback = function(e) {
 	testFailed("Should not get here.");
 };