Make dump_tree.js tolerate nodes without a .children property.

Possibly a bug in our implementation, but some objects seen
from JS do not expose children/parent properties (since they
subclass CJX_Object rather than CJX_Node).

Change-Id: Ia5098539870131667a1abe55c6365105794aeb96
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/66270
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>

testing/resources/javascript/xfa_specific/dump_tree.js

diff --git a/testing/resources/javascript/xfa_specific/dump_tree.js b/testing/resources/javascript/xfa_specific/dump_tree.js
index a82aec6..0befd6b 100644
--- a/testing/resources/javascript/xfa_specific/dump_tree.js
+++ b/testing/resources/javascript/xfa_specific/dump_tree.js
@@ -11,8 +11,10 @@
   try {
     app.alert(indentation + node.className);
     var children = node.nodes;
-    for (var i = 0; i < children.length; ++i) {
-      dumpTree(children.item(i), level + 1);
+    if (children) {
+      for (var i = 0; i < children.length; ++i) {
+        dumpTree(children.item(i), level + 1);
+      }
     }
   } catch (e) {
     app.alert(indentation + "Error: " + e);