Fix too many parens in @supports serialization.

tests/unit/style/parsing/supports.rs

@@ -0,0 +1,14 @@
+/* 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 style::supports::SupportsCondition;
+use style_traits::ToCss;
+
+#[test]
+fn test_supports_condition() {
+    assert_roundtrip!(SupportsCondition::parse, "(margin: 1px)");
+    assert_roundtrip!(SupportsCondition::parse, "not (--be: to be)");
+    assert_roundtrip!(SupportsCondition::parse, "(color: blue) and future-extension(4)");
+}