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/shared/embedder/lib.rs

@@ -21,7 +21,7 @@ use std::ops::Range;
 use std::path::PathBuf;
 use std::sync::Arc;
 
-use base::generic_channel::GenericSender;
+use base::generic_channel::{GenericCallback, GenericSender, SendResult};
 use base::id::{PipelineId, WebViewId};
 use crossbeam_channel::Sender;
 use euclid::{Point2D, Scale, Size2D};
@@ -1081,3 +1081,34 @@ pub struct RgbColor {
     pub green: u8,
     pub blue: u8,
 }
+
+/// A Script to Embedder Channel
+#[derive(Clone, Debug, Deserialize, MallocSizeOf, Serialize)]
+pub struct ScriptToEmbedderChan(GenericCallback<EmbedderMsg>);
+
+impl ScriptToEmbedderChan {
+    /// Create a new Channel allowing script to send messages to the Embedder
+    pub fn new(
+        embedder_chan: Sender<EmbedderMsg>,
+        waker: Box<dyn EventLoopWaker>,
+    ) -> ScriptToEmbedderChan {
+        let embedder_callback = GenericCallback::new(move |embedder_msg| {
+            let msg = match embedder_msg {
+                Ok(embedder_msg) => embedder_msg,
+                Err(err) => {
+                    log::warn!("Script to Embedder message error: {err}");
+                    return;
+                },
+            };
+            let _ = embedder_chan.send(msg);
+            waker.wake();
+        })
+        .expect("Failed to create channel");
+        ScriptToEmbedderChan(embedder_callback)
+    }
+
+    /// Send a message to and wake the Embedder
+    pub fn send(&self, msg: EmbedderMsg) -> SendResult {
+        self.0.send(msg)
+    }
+}