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/script/Cargo.toml

@@ -76,7 +76,6 @@ media = { path = "../media" }
 metrics = { path = "../metrics" }
 mime = { workspace = true }
 mime_guess = { workspace = true }
-mitochondria = { workspace = true }
 msg = { path = "../msg" }
 net_traits = { path = "../net_traits" }
 num-traits = { workspace = true }

components/script/dom/bindings/root.rs

@@ -33,9 +33,8 @@ use crate::dom::node::Node;
 use js::jsapi::{Heap, JSObject, JSTracer};
 use js::rust::GCMethods;
 use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
-use mitochondria::OnceCell;
 use script_layout_interface::TrustedNodeAddress;
-use std::cell::{Cell, UnsafeCell};
+use std::cell::{Cell, OnceCell, UnsafeCell};
 use std::default::Default;
 use std::hash::{Hash, Hasher};
 use std::marker::PhantomData;
@@ -718,7 +717,7 @@ where
         F: FnOnce() -> DomRoot<T>,
     {
         assert_in_script();
-        &self.ptr.init_once(|| Dom::from_ref(&cb()))
+        &self.ptr.get_or_init(|| Dom::from_ref(&cb()))
     }
 }
 
@@ -742,7 +741,7 @@ impl<T: DomObject> MallocSizeOf for DomOnceCell<T> {
 #[allow(unrooted_must_root)]
 unsafe impl<T: DomObject> JSTraceable for DomOnceCell<T> {
     unsafe fn trace(&self, trc: *mut JSTracer) {
-        if let Some(ptr) = self.ptr.as_ref() {
+        if let Some(ptr) = self.ptr.get() {
             ptr.trace(trc);
         }
     }

components/script/lib.rs

@@ -4,6 +4,7 @@
 
 #![feature(core_intrinsics)]
 #![feature(drain_filter)]
+#![feature(once_cell)]
 #![feature(plugin)]
 #![feature(register_tool)]
 #![deny(unsafe_code)]