Reject negative values of flex-basis.

Fixes #15902

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 {