Auto merge of #15869 - Iakis:NegBordRad, r=Wafflespeanut

Reject negative border radius

Replacerd Parse in parse_one_set_of_border_values with parse_non_negative and added tests for it
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- [ X] `./mach test-tidy` does not report any errors
- [X ] These changes fix #15345 (github issue number if applicable).

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- [ ] These changes do not require tests because _____

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components/style/values/specified/basic_shape.rs

@@ -749,10 +749,10 @@ impl ToCss for BorderRadius {
 }
 
 impl Parse for BorderRadius {
-    fn parse(context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
-        let mut widths = try!(parse_one_set_of_border_values(context, input));
+    fn parse(_context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
+        let mut widths = try!(parse_one_set_of_border_values(input));
         let mut heights = if input.try(|input| input.expect_delim('/')).is_ok() {
-            try!(parse_one_set_of_border_values(context, input))
+            try!(parse_one_set_of_border_values(input))
         } else {
             [widths[0].clone(),
              widths[1].clone(),
@@ -768,23 +768,22 @@ impl Parse for BorderRadius {
     }
 }
 
-fn parse_one_set_of_border_values(context: &ParserContext, mut input: &mut Parser)
+fn parse_one_set_of_border_values(mut input: &mut Parser)
                                  -> Result<[LengthOrPercentage; 4], ()> {
-    let a = try!(LengthOrPercentage::parse(context, input));
-
-    let b = if let Ok(b) = input.try(|i| LengthOrPercentage::parse(context, i)) {
+    let a = try!(LengthOrPercentage::parse_non_negative(input));
+    let b = if let Ok(b) = input.try(|i| LengthOrPercentage::parse_non_negative(i)) {
         b
     } else {
         return Ok([a.clone(), a.clone(), a.clone(), a])
     };
 
-    let c = if let Ok(c) = input.try(|i| LengthOrPercentage::parse(context, i)) {
+    let c = if let Ok(c) = input.try(|i| LengthOrPercentage::parse_non_negative(i)) {
         c
     } else {
         return Ok([a.clone(), b.clone(), a, b])
     };
 
-    if let Ok(d) = input.try(|i| LengthOrPercentage::parse(context, i)) {
+    if let Ok(d) = input.try(|i| LengthOrPercentage::parse_non_negative(i)) {
         Ok([a, b, c, d])
     } else {
         Ok([a, b.clone(), c, b])