Auto merge of #13965 - asajeffrey:script-iframe-check-document-browsing-context, r=Ms2ger

Check that an iframe is in a document with a browsing context before processing src

<!-- Please describe your changes on the following line: -->

Check that an iframe is in a document with a browsing context before processing src.
---

<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #13964.
- [X] These changes do not require tests because this is already tested by https://github.com/servo/servo/blob/master/tests/wpt/web-platform-tests/old-tests/submission/Opera/script_scheduling/034.html

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---

This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/13965)

<!-- Reviewable:end -->

components/script/dom/node.rs

@@ -771,6 +771,10 @@ impl Node {
         self.owner_doc().is_html_document()
     }
 
+    pub fn is_in_doc_with_browsing_context(&self) -> bool {
+        self.is_in_doc() && self.owner_doc().browsing_context().is_some()
+    }
+
     pub fn children(&self) -> NodeSiblingIterator {
         NodeSiblingIterator {
             current: self.GetFirstChild(),