debugmozjs build is broken

[jdm]

servo/components/script/script_runtime.rs

Lines 682 to 696 in ba92920

	#[allow(unsafe_code)]
	#[cfg(feature = "debugmozjs")]
	unsafe fn set_gc_zeal_options(cx: *mut JSContext) {
	use js::jsapi::{JS_SetGCZeal, JS_DEFAULT_ZEAL_FREQ};
	let level = match pref!(js.mem.gc.zeal.level) {
	Some(level @ 0...14) => level as u8,
	_ => return,
	};
	let frequency = match pref!(js.mem.gc.zeal.frequency) {
	Some(frequency) if frequency >= 0 => frequency as u32,
	_ => JS_DEFAULT_ZEAL_FREQ,
	};
	JS_SetGCZeal(cx, level, frequency);
	}

is not built by default and doesn't compile since #22923.