Auto merge of #13541 - canaltinova:origin-clip, r=Manishearth

Fix origin/clip parsing behavior in mask and background shorthands

<!-- Please describe your changes on the following line: -->
We have tests for mask shorthand parsing, but we don't have tests for background parsing. Should I add some for it?
Also deleted inaccessible match arms in serialization function.
r? @Manishearth

---
<!-- 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 #13466 (github issue number if applicable).

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

<!-- 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/13541)
<!-- Reviewable:end -->

tests/unit/style/parsing/mask.rs

@@ -106,8 +106,8 @@ fn mask_shorthand_should_parse_origin_and_clip_correctly() {
     let mut parser = Parser::new("padding-box");
     let result = mask::parse_value(&context, &mut parser).unwrap();
 
-    // TODO(#13466): We should fix origin/clip parsing behavior.
     assert_eq!(result.mask_origin.unwrap(), parse_longhand!(mask_origin, "padding-box"));
+    assert_eq!(result.mask_clip.unwrap(), parse_longhand!(mask_clip, "padding-box"));
 }
 
 #[test]

tests/wpt/metadata-css/css-backgrounds-3_dev/html4/background-335.htm.ini

@@ -1,5 +0,0 @@
-[background-335.htm]
-  type: testharness
-  [background_specified_box_one_clip]
-    expected: FAIL
-