Auto merge of #15237 - samuknet:negative-letter-spacing-word-spacing, r=emilio

Allow negative values for letter-spacing and word-spacing.  Inline Le…

<!-- Please describe your changes on the following line: -->
Allow negative values when parsing `letter-spacing` and `word-spacing`.  Inline `parse_non_negative` in `Length`.

---
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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 #15204 (github issue number if applicable).

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- [x] There are tests for these changes

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

@@ -335,11 +335,11 @@ ${helpers.single_keyword("text-align-last",
         }
     }
 
-    pub fn parse(_context: &ParserContext, input: &mut Parser) -> Result<SpecifiedValue, ()> {
+    pub fn parse(context: &ParserContext, input: &mut Parser) -> Result<SpecifiedValue, ()> {
         if input.try(|input| input.expect_ident_matching("normal")).is_ok() {
             Ok(SpecifiedValue::Normal)
         } else {
-            specified::Length::parse_non_negative(input).map(SpecifiedValue::Specified)
+            specified::Length::parse(context, input).map(SpecifiedValue::Specified)
         }
     }
 </%helpers:longhand>
@@ -416,11 +416,11 @@ ${helpers.single_keyword("text-align-last",
         }
     }
 
-    pub fn parse(_context: &ParserContext, input: &mut Parser) -> Result<SpecifiedValue, ()> {
+    pub fn parse(context: &ParserContext, input: &mut Parser) -> Result<SpecifiedValue, ()> {
         if input.try(|input| input.expect_ident_matching("normal")).is_ok() {
             Ok(SpecifiedValue::Normal)
         } else {
-            specified::LengthOrPercentage::parse_non_negative(input)
+            specified::LengthOrPercentage::parse(context, input)
                                           .map(SpecifiedValue::Specified)
         }
     }

components/style/values/specified/length.rs

@@ -463,6 +463,7 @@ impl Length {
     }
 
     /// Parse a non-negative length
+    #[inline]
     pub fn parse_non_negative(input: &mut Parser) -> Result<Length, ()> {
         Length::parse_internal(input, AllowedNumericType::NonNegative)
     }