Auto merge of #18344 - bradwerth:selectorSerialize, r=SimonSapin

Change selector::to_css function to handle combinators in between universal selectors.

https://bugzilla.mozilla.org/show_bug.cgi?id=1391169
https://reviewboard.mozilla.org/r/172368/

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tests/unit/style/parsing/selectors.rs

@@ -25,4 +25,6 @@ fn test_selectors() {
     assert_roundtrip!(parse_selector, "svg|circle");
     assert_roundtrip!(parse_selector, "p:before", "p::before");
     assert_roundtrip!(parse_selector, "[border=\"0\"]:-servo-nonzero-border ~ ::-servo-details-summary");
+    assert_roundtrip!(parse_selector, "* > *");
+    assert_roundtrip!(parse_selector, "*|* + *", "* + *");
 }