Add ShapeSource to basic-shape values

components/style/parser.rs

@@ -81,3 +81,11 @@ impl<'a> ParserContext<'a> {
 pub fn log_css_error(input: &mut Parser, position: SourcePosition, message: &str, parsercontext: &ParserContext) {
     parsercontext.error_reporter.report_error(input, position, message);
 }
+
+// XXXManishearth Replace all specified value parse impls with impls of this
+// trait. This will make it easy to write more generic values in the future.
+// There may need to be two traits -- one for parsing with context, and one
+// for parsing without
+pub trait Parse {
+    fn parse(input: &mut Parser) -> Result<Self, ()> where Self: Sized;
+}