Auto merge of #21051 - jonathanKingston:window-length, r=jdm

Implement window.length DOM attribute.

Implements the `window.length` DOM attribute for enumerating frames on the document.

---
<!-- 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

<!-- Either: -->
- [X] There are tests for these changes

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- 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/21051)
<!-- Reviewable:end -->

tests/wpt/metadata/html/browsers/the-window-object/accessing-other-browsing-contexts/window_length.html.ini

@@ -1,19 +1,5 @@
 [window_length.html]
   type: testharness
-  [No child browsing contexts]
-    expected: FAIL
-
-  [iframe not inserted into the document]
-    expected: FAIL
-
-  [One iframe inserted into the document]
-    expected: FAIL
-
-  [Child browsing context has a child browsing context]
-    expected: FAIL
-
-  [window.length in child frame]
-    expected: FAIL
 
   [Opened window]
     expected: FAIL

tests/wpt/metadata/html/browsers/the-window-object/security-window/window-security.https.html.ini

@@ -335,9 +335,6 @@
   [A SecurityError exception must be thrown when window.stop is accessed from a different origin.]
     expected: FAIL
 
-  [A SecurityError exception should not be thrown when window.length is accessed from a different origin.]
-    expected: FAIL
-
   [A SecurityError exception should not be thrown when window.opener is accessed from a different origin.]
     expected: FAIL
 

tests/wpt/metadata/html/browsers/the-window-object/window-properties.https.html.ini

@@ -53,9 +53,6 @@
   [Window replaceable attribute: external]
     expected: FAIL
 
-  [Window replaceable attribute: length]
-    expected: FAIL
-
   [Window replaceable attribute: screen]
     expected: FAIL