godoc: fix bug in "internal call graph" tree widget.

Today I learned that getElementByClass returns an
HTMLContainer, not an array, so the for loop was iterating its
properties, which include:
- the methods of the type, which were filtered out by the condition;
- the array-like integer indices of the elements, which we want;
- the ids of the same elements (!), which we weren't expecting.
The net result is that various functions were called twice,
causing the tree to be double-populated and clicks to cause
and expand+collapse.

It's a miracle to me that any JS program even approximately works.

Fixes golang/go#8237

LGTM=bradfitz
R=gri, bradfitz
CC=golang-codereviews
https://golang.org/cl/141540043

godoc/static/godocs.js

@@ -441,7 +441,7 @@ function setupCallgraphs() {
   document.getElementById("pkg-callgraph").style.display = "block";
 
   var treeviews = document.getElementsByClassName("treeview");
-  for (var i in treeviews) {
+  for (var i = 0; i < treeviews.length; i++) {
     var tree = treeviews[i];
     if (tree.id == null || tree.id.indexOf("callgraph-") != 0) {
       continue;

godoc/static/static.go

@@ -1014,6 +1014,7 @@ function setupCallgraphs() {
   document.getElementById("pkg-callgraph").style.display = "block";
 
   var treeviews = document.getElementsByClassName("treeview");
+  //  for (var i = 0; i < treeviews.length; i++) {
   for (var i in treeviews) {
     var tree = treeviews[i];
     if (tree.id == null || tree.id.indexOf("callgraph-") != 0) {