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/constellation/lib.rs

@@ -16,7 +16,6 @@ mod pipeline;
 mod sandboxing;
 mod serviceworker;
 mod session_history;
-mod timer_scheduler;
 mod webview;
 
 pub use crate::constellation::{Constellation, InitialConstellationState};