Auto merge of #23153 - 5h1rU:xmlserializer-serializetostring-panics, r=jdm

Make serializeToString serialize document nodes correctly

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This is the fix for ScriptThread panic when `new XMLSerializer().serializeToString(document);` is called.

r?@jdm

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- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #23130

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components/script/dom/servoparser/html.rs

@@ -167,7 +167,7 @@ fn rev_children_iter(n: &Node) -> impl Iterator<Item = DomRoot<Node>> {
 impl SerializationIterator {
     fn new(node: &Node, skip_first: bool) -> SerializationIterator {
         let mut ret = SerializationIterator { stack: vec![] };
-        if skip_first || node.is::<DocumentFragment>() {
+        if skip_first || node.is::<DocumentFragment>() || node.is::<Document>() {
             for c in rev_children_iter(node) {
                 ret.push_node(&*c);
             }