Make the unrooted_must_root conditional on a default Cargo feature.

Only http://perf.rust-lang.org/ will disable it,
in order to be less subject to changes to rustc internal APIs.

components/script/Cargo.toml

@@ -14,6 +14,8 @@ path = "lib.rs"
 [features]
 debugmozjs = ['js/debugmozjs']
 unstable = ["servo_allocator/unstable"]
+unrooted_must_root_lint = ["script_plugins/unrooted_must_root_lint"]
+default = ["unrooted_must_root_lint"]
 
 [build-dependencies]
 cmake = "0.1"