Auto merge of #16157 - n0max:perspective_parse_non_negative, r=emilio

Parse perspective property as non negative and add tests

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15449  (github issue number if applicable).

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- [X] There are tests for these changes

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components/style/properties/longhand/box.mako.rs

@@ -1778,6 +1778,7 @@ ${helpers.single_keyword("resize",
 ${helpers.predefined_type("perspective",
                           "LengthOrNone",
                           "Either::Second(None_)",
+                          "parse_non_negative_length",
                           gecko_ffi_name="mChildPerspective",
                           spec="https://drafts.csswg.org/css-transforms/#perspective",
                           extra_prefixes="moz webkit",

components/style/values/specified/length.rs

@@ -466,6 +466,17 @@ impl Parse for Length {
     }
 }
 
+impl Either<Length, None_> {
+    /// Parse a non-negative length or none
+    #[inline]
+    pub fn parse_non_negative_length(context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
+        if input.try(|input| None_::parse(context, input)).is_ok() {
+            return Ok(Either::Second(None_));
+        }
+        Length::parse_non_negative(input).map(Either::First)
+    }
+}
+
 impl Either<Length, Normal> {
     #[inline]
     #[allow(missing_docs)]