doc: don't block page load on JavaScript fetch

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/12050045
diff --git a/doc/codewalk/codewalk.js b/doc/codewalk/codewalk.js
index eb4667a..7bfcd39 100644
--- a/doc/codewalk/codewalk.js
+++ b/doc/codewalk/codewalk.js
@@ -296,7 +296,7 @@
   this.sizer.height(codeHeight);
 };
 
-jQuery(document).ready(function() {
+window.initFuncs.push(function() {
   var viewer = new CodewalkViewer(jQuery('#codewalk-main'));
   viewer.selectFirstComment();
   viewer.targetCommentLinksAtBlank();