Auto merge of #15820 - snehasi:master, r=KiChjang

border-image no more accepting shorthand values

Fixes #15770

Added a check in border.mako.rs, if `w` and `o` are not none before returning Ok status

---
- [x] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15770

- [ ] There are tests for these changes OR
- [X] These changes do not require tests because they fix the previous errors reported in #15770

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components/style/properties/shorthand/border.mako.rs

@@ -246,7 +246,12 @@ pub fn parse_border(context: &ParserContext, input: &mut Parser)
                                 try!(input.expect_delim('/'));
                                 border_image_outset::parse(context, input)
                             }).ok();
-                            Ok((w, o))
+                            if w.is_none() && o.is_none() {
+                               Err(())
+                            }
+                            else {
+                               Ok((w, o))
+                            }
                         });
                         if let Ok((w, o)) = maybe_width_outset {
                             width = w;