use ::parse_non_negative instead of ::parse for background_size parsing

tests/unit/style/properties/background.rs

@@ -0,0 +1,19 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use cssparser::Parser;
+use media_queries::CSSErrorReporterTest;
+use style::parser::ParserContext;
+use style::properties::longhands::background_size;
+use style::stylesheets::Origin;
+
+#[test]
+fn background_size_should_reject_negative_values() {
+    let url = ::servo_url::ServoUrl::parse("http://localhost").unwrap();
+    let context = ParserContext::new(Origin::Author, &url, Box::new(CSSErrorReporterTest));
+
+    let parse_result = background_size::parse(&context, &mut Parser::new("-40% -40%"));
+
+    assert_eq!(parse_result.is_err(), true);
+}

tests/unit/style/properties/mod.rs

@@ -2,6 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+mod background;
 mod scaffolding;
 mod serialization;
 mod viewport;