script: Use enums for event loop senders and receivers (#34896)

Previously, senders and receivers to different kinds of event loops (the
main `ScriptThread`, different types of workers) used a rust `trait`
mechanism to implement dynamic behavior. This led to having many unused
implementations of this `trait`. This change moves to using an `enum`
based approach for these senders and receivers and removes all of the
dead code.

In addition, to allowing for use of rust's dead code detection, it
simplifies the code a great deal. All of these generic senders and
receivers are moved to the `messaging.rs` file and given proper
documentation.

Finally, empty an `JSTraceable` implementation is made for all
crossbeam `Sender<...>`s to avoid having to manually skip them everytime
they are included in structs. The pre-existing empty `MallocSizeOf`
implementation is used more thoroughly.

Other unecessary wrappers around these senders and receivers are removed
as well.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/bindings/refcounted.rs

@@ -53,7 +53,10 @@ mod dummy {
 pub use self::dummy::LIVE_REFERENCES;
 
 /// A pointer to a Rust DOM object that needs to be destroyed.
-struct TrustedReference(*const libc::c_void);
+#[derive(MallocSizeOf)]
+struct TrustedReference(
+    #[ignore_malloc_size_of = "This is a shared reference."] *const libc::c_void,
+);
 unsafe impl Send for TrustedReference {}
 
 impl TrustedReference {
@@ -158,10 +161,13 @@ impl TrustedPromise {
 /// DOM object is guaranteed to live at least as long as the last outstanding
 /// `Trusted<T>` instance.
 #[crown::unrooted_must_root_lint::allow_unrooted_interior]
+#[derive(MallocSizeOf)]
 pub struct Trusted<T: DomObject> {
     /// A pointer to the Rust DOM object of type T, but void to allow
     /// sending `Trusted<T>` between threads, regardless of T's sendability.
+    #[conditional_malloc_size_of]
     refcount: Arc<TrustedReference>,
+    #[ignore_malloc_size_of = "These are shared by all `Trusted` types."]
     owner_thread: *const LiveDOMReferences,
     phantom: PhantomData<T>,
 }