[release-branch.go1.2] go-tour: add analytics to old tour.

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/135840043
diff --git a/js/tour.js b/js/tour.js
index e1f113d..1c1424a 100644
--- a/js/tour.js
+++ b/js/tour.js
@@ -192,6 +192,9 @@
 	slide = slides[i];
 	var $s = $(slide).show();
 
+	// notify the change of slides if analytics is available.
+	if (window.trackPageview) window.trackPageview();
+
 	// load stored code, or hide code box
 	if ($s.hasClass("nocode")) {
 		$('#workspace').hide();