Use std::cell::OnceCell and remove mitochondria dependency (#30207)

`OnceCell` is now part of the standard library and we'll be able to use
it once we upgrade rust. For now we can use the version that's shipped
behind a feature flag in rust. This removes a dependency on one crate.

components/layout_2020/lib.rs

@@ -3,6 +3,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 #![deny(unsafe_code)]
+#![feature(once_cell)]
 
 #[macro_use]
 extern crate log;