Auto merge of #18875 - servo:stable-js, r=nox,jdm

Remove the need for rust-mozjs to use unstable Rust features

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tests/unit/style/lib.rs

@@ -3,7 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #![cfg(test)]
-#![feature(plugin, test)]
+#![feature(test)]
 
 extern crate app_units;
 extern crate cssparser;