script: Make timers per-process (#34581)

Before all timers were managed by the Constellation process, meaning
that they had to trigger IPC calls to be scheduled and fired. Currently,
timers are only used in the `ScriptThread`, so it makes sense that they
are per-process.

This change restores the timer thread functionality that existed before
avoided entirely. Completion is done using a callback that is sent to
the timer thread similarly to how fetch is done. This allows reusing the
existing task queue without making any new channels.

Fixes #15219.

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

components/malloc_size_of/lib.rs

@@ -46,6 +46,8 @@
 //!   Note: WebRender has a reduced fork of this crate, so that we can avoid
 //!   publishing this crate on crates.io.
 
+use std::cell::OnceCell;
+use std::collections::BinaryHeap;
 use std::hash::{BuildHasher, Hash};
 use std::ops::Range;
 
@@ -351,6 +353,12 @@ impl<T: MallocSizeOf> MallocSizeOf for thin_vec::ThinVec<T> {
     }
 }
 
+impl<T: MallocSizeOf> MallocSizeOf for BinaryHeap<T> {
+    fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
+        self.iter().map(|element| element.size_of(ops)).sum()
+    }
+}
+
 macro_rules! malloc_size_of_hash_set {
     ($ty:ty) => {
         impl<T, S> MallocShallowSizeOf for $ty
@@ -468,6 +476,14 @@ impl<T> MallocSizeOf for std::marker::PhantomData<T> {
     }
 }
 
+impl<T: MallocSizeOf> MallocSizeOf for OnceCell<T> {
+    fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
+        self.get()
+            .map(|interior| interior.size_of(ops))
+            .unwrap_or_default()
+    }
+}
+
 // See https://github.com/rust-lang/rust/issues/68318:
 // We don't want MallocSizeOf to be defined for Rc and Arc. If negative trait bounds are
 // ever allowed, this code should be uncommented.  Instead, there is a compile-fail test for