Auto merge of #16274 - CarlosMcs:negative-flex-basis, r=emilio

Reject negative values of flex-basis.

Fixes #15902

<!-- 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 #15902 (github issue number if applicable).

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

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- 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/16274)
<!-- Reviewable:end -->

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

@@ -142,6 +142,7 @@ ${helpers.predefined_type("flex-basis",
                           "LengthOrPercentageOrAutoOrContent",
                           "computed::LengthOrPercentageOrAuto::Auto" if product == "gecko" else
                           "computed::LengthOrPercentageOrAutoOrContent::Auto",
+                          "parse_non_negative_with_context",
                           spec="https://drafts.csswg.org/css-flexbox/#flex-basis-property",
                           extra_prefixes="webkit",
                           animatable=True if product == "gecko" else False)}

components/style/values/specified/length.rs

@@ -1170,6 +1170,14 @@ impl LengthOrPercentageOrAuto {
     pub fn parse_non_negative(input: &mut Parser) -> Result<LengthOrPercentageOrAuto, ()> {
         LengthOrPercentageOrAuto::parse_internal(input, AllowedNumericType::NonNegative)
     }
+
+    /// Parse a non-negative length, percentage, or auto.
+    #[inline]
+    pub fn parse_non_negative_with_context(_context: &ParserContext,
+                                           input: &mut Parser)
+                                           -> Result<LengthOrPercentageOrAuto, ()> {
+        LengthOrPercentageOrAuto::parse_non_negative(input)
+    }
 }
 
 impl Parse for LengthOrPercentageOrAuto {
@@ -1273,6 +1281,16 @@ pub enum LengthOrPercentageOrAutoOrContent {
     Content
 }
 
+impl LengthOrPercentageOrAutoOrContent {
+    /// Alias to `parse` so that Gecko and Servo can use the same method name for
+    /// both `LengthOrPercentageOrAuto` and `LengthOrPercentageOrAutoOrContent`.
+    ///
+    /// NOTE: `parse` already only accepts non-negative values.
+    pub fn parse_non_negative_with_context(context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
+        Self::parse(context, input)
+    }
+}
+
 impl HasViewportPercentage for LengthOrPercentageOrAutoOrContent {
     fn has_viewport_percentage(&self) -> bool {
         match *self {