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/globalscope.rs

@@ -26,7 +26,7 @@ use content_security_policy::CspList;
 use crossbeam_channel::Sender;
 use devtools_traits::{PageError, ScriptToDevtoolsControlMsg};
 use dom_struct::dom_struct;
-use embedder_traits::{EmbedderMsg, JavaScriptEvaluationError};
+use embedder_traits::{EmbedderMsg, JavaScriptEvaluationError, ScriptToEmbedderChan};
 use fonts::FontContext;
 use ipc_channel::ipc::{self, IpcSender};
 use ipc_channel::router::ROUTER;
@@ -254,6 +254,11 @@ pub(crate) struct GlobalScope {
     #[no_trace]
     script_to_constellation_chan: ScriptToConstellationChan,
 
+    /// A handle for communicating messages to the Embedder.
+    #[ignore_malloc_size_of = "channels are hard"]
+    #[no_trace]
+    script_to_embedder_chan: ScriptToEmbedderChan,
+
     /// <https://html.spec.whatwg.org/multipage/#in-error-reporting-mode>
     in_error_reporting_mode: Cell<bool>,
 
@@ -741,6 +746,7 @@ impl GlobalScope {
         mem_profiler_chan: profile_mem::ProfilerChan,
         time_profiler_chan: profile_time::ProfilerChan,
         script_to_constellation_chan: ScriptToConstellationChan,
+        script_to_embedder_chan: ScriptToEmbedderChan,
         resource_threads: ResourceThreads,
         origin: MutableOrigin,
         creation_url: ServoUrl,
@@ -770,6 +776,7 @@ impl GlobalScope {
             mem_profiler_chan,
             time_profiler_chan,
             script_to_constellation_chan,
+            script_to_embedder_chan,
             in_error_reporting_mode: Default::default(),
             resource_threads,
             timers: OnceCell::default(),
@@ -2477,8 +2484,12 @@ impl GlobalScope {
         &self.script_to_constellation_chan
     }
 
+    pub(crate) fn script_to_embedder_chan(&self) -> &ScriptToEmbedderChan {
+        &self.script_to_embedder_chan
+    }
+
     pub(crate) fn send_to_embedder(&self, msg: EmbedderMsg) {
-        self.send_to_constellation(ScriptToConstellationMessage::ForwardToEmbedder(msg));
+        self.script_to_embedder_chan().send(msg).unwrap();
     }
 
     pub(crate) fn send_to_constellation(&self, msg: ScriptToConstellationMessage) {