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/script_thread.rs

@@ -781,6 +781,10 @@ impl ScriptThread {
                             .senders
                             .pipeline_to_constellation_sender
                             .clone(),
+                        to_embedder_sender: script_thread
+                            .senders
+                            .pipeline_to_embedder_sender
+                            .clone(),
                         image_cache,
                         #[cfg(feature = "webgpu")]
                         gpu_id_hub: script_thread.gpu_id_hub.clone(),
@@ -941,6 +945,7 @@ impl ScriptThread {
             bluetooth_sender: state.bluetooth_sender,
             constellation_sender: state.constellation_sender,
             pipeline_to_constellation_sender: state.pipeline_to_constellation_sender.sender.clone(),
+            pipeline_to_embedder_sender: state.pipeline_to_embedder_sender.clone(),
             image_cache_sender: ipc_image_cache_sender,
             time_profiler_sender: state.time_profiler_sender,
             memory_profiler_sender: state.memory_profiler_sender,
@@ -966,6 +971,7 @@ impl ScriptThread {
             senders.memory_profiler_sender.clone(),
             senders.time_profiler_sender.clone(),
             script_to_constellation_chan,
+            senders.pipeline_to_embedder_sender.clone(),
             state.resource_threads.clone(),
             #[cfg(feature = "webgpu")]
             gpu_id_hub.clone(),
@@ -3312,6 +3318,7 @@ impl ScriptThread {
             self.senders.time_profiler_sender.clone(),
             self.senders.devtools_server_sender.clone(),
             script_to_constellation_chan,
+            self.senders.pipeline_to_embedder_sender.clone(),
             self.senders.constellation_sender.clone(),
             incomplete.pipeline_id,
             incomplete.parent_info,