Auto merge of #12416 - canaltinova:referrer, r=jdm

Implement Document.referrer

<!-- Please describe your changes on the following line: -->
---
<!-- 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 #12389 (github issue number if applicable).

<!-- Either: -->
- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- 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/12416)
<!-- Reviewable:end -->

tests/wpt/metadata/html/dom/interfaces.html.ini

@@ -3,9 +3,6 @@
   [Document interface: attribute domain]
     expected: FAIL
 
-  [Document interface: attribute referrer]
-    expected: FAIL
-
   [Document interface: attribute dir]
     expected: FAIL
 
@@ -6987,9 +6984,6 @@
   [Document interface: document.implementation.createDocument(null, "", null) must inherit property "enableStyleSheetsForSet" with the proper type (33)]
     expected: FAIL
 
-  [Document interface: document.implementation.createDocument(null, "", null) must inherit property "referrer" with the proper type (36)]
-    expected: FAIL
-
   [Document interface: document.implementation.createDocument(null, "", null) must inherit property "dir" with the proper type (42)]
     expected: FAIL
 
@@ -7347,9 +7341,6 @@
   [Document interface: new Document() must have own property "location"]
     expected: FAIL
 
-  [Document interface: new Document() must inherit property "referrer" with the proper type (36)]
-    expected: FAIL
-
   [Document interface: new Document() must inherit property "dir" with the proper type (42)]
     expected: FAIL