tour: kill running code snippets when changing page

When a new page is loaded, the backend is told to kill still running code snippets,
known to the frontend at this point.

Fixes golang/go#16418

Change-Id: Ie4c246b6bf9f566ef7523e35b44395a1abfd4e41
Reviewed-on: https://go-review.googlesource.com/98675
Reviewed-by: Andrew Bonventre <andybons@golang.org>
diff --git a/static/js/controllers.js b/static/js/controllers.js
index c64fb04..f3b9cf7 100755
--- a/static/js/controllers.js
+++ b/static/js/controllers.js
@@ -47,6 +47,7 @@
             $scope.gotoPage($scope.curPage - 1);
         };
         $scope.gotoPage = function(page) {
+            $scope.kill();
             var l = $routeParams.lessonId;
             if (page >= 1 && page <= lessons[$scope.lessonId].Pages.length) {
                 $scope.curPage = page;