Add direct script to embedder channel (#39039)

This PR **removes** `ScriptToConstellationMessage::ForwardToEmbedder`,
and replaces it with an explicit `ScriptToEmbedderChannel`. This new
channel is based on `GenericCallback` and in single-process mode will
directly send the message the to the embedder and wake it. In
multi-process mode, the message is routed via the ROUTER, since waking
is only possible from the same process currently. This means in
multi-process mode there are likely no direct perf benefits, since we
still need to hop the message over the ROUTER (instead of over the
constellation).
In single-process mode we can directly send the message to the embedder,
which should provide a noticable latency improvement in all cases where
script is blocked waiting on the embedder to reply.

This does not change the way the embedder receives messages - the
receiving end is unchanged.

## How was sending messages to the embedder working before?

1. Script wraps it's message to the embedder in
`ScriptToConstellationMessage::ForwardToEmbedder` and sends it to
constellation.
2. The [constellation event loop] receives the message in
[handle_request]
3. If deserialization fails, [an error is logged and the message is
ignored]
4. Since our message came from script, it is handle in
[handle_request_from_script]
5. The message is logged with trace log level
6. If the pipeline is closed, [a warning is logged and the message
ignored]
7. The wrapped `EmbedderMsg` [is forwarded to the embedder]. Sending the
message also invokes `wake()` on the embedder eventloop waker.

[constellation event loop]:
2e1b2e7260/components/constellation/constellation.rs (L755)

[handle request]:
2e1b2e7260/components/constellation/constellation.rs (L1182)

[an error is logged and the message is ignored]:
2e1b2e7260/components/constellation/constellation.rs (L1252)

[handle_request_from_script]:
https://github.com/servo/servo/blob/main/components/constellation/constellation.rs#L1590
 
[a warning is logged and the message ignored]:
2e1b2e7260/components/constellation/constellation.rs (L1599)

[is forwarded to the embedder]:
2e1b2e7260/components/constellation/constellation.rs (L1701)

Testing: Communication between Script and Embedder is extensive, so this
should be covered by existing tests.

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

components/script/dom/workletglobalscope.rs

@@ -10,7 +10,7 @@ use constellation_traits::{ScriptToConstellationChan, ScriptToConstellationMessa
 use crossbeam_channel::Sender;
 use devtools_traits::ScriptToDevtoolsControlMsg;
 use dom_struct::dom_struct;
-use embedder_traits::JavaScriptEvaluationError;
+use embedder_traits::{JavaScriptEvaluationError, ScriptToEmbedderChan};
 use ipc_channel::ipc::IpcSender;
 use js::jsval::UndefinedValue;
 use net_traits::ResourceThreads;
@@ -100,6 +100,7 @@ impl WorkletGlobalScope {
                 init.mem_profiler_chan.clone(),
                 init.time_profiler_chan.clone(),
                 script_to_constellation_chan,
+                init.to_embedder_sender.clone(),
                 init.resource_threads.clone(),
                 MutableOrigin::new(ImmutableOrigin::new_opaque()),
                 base_url.clone(),
@@ -198,6 +199,8 @@ pub(crate) struct WorkletGlobalScopeInit {
     pub(crate) devtools_chan: Option<IpcSender<ScriptToDevtoolsControlMsg>>,
     /// Messages to send to constellation
     pub(crate) to_constellation_sender: GenericSender<(PipelineId, ScriptToConstellationMessage)>,
+    /// Messages to send to the Embedder
+    pub(crate) to_embedder_sender: ScriptToEmbedderChan,
     /// The image cache
     pub(crate) image_cache: Arc<dyn ImageCache>,
     /// Identity manager for WebGPU resources