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/clipboard_provider.rs

@@ -3,8 +3,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use base::id::WebViewId;
-use constellation_traits::{ScriptToConstellationChan, ScriptToConstellationMessage};
-use embedder_traits::EmbedderMsg;
+use embedder_traits::{EmbedderMsg, ScriptToEmbedderChan};
 use ipc_channel::ipc::channel;
 use malloc_size_of_derive::MallocSizeOf;
 
@@ -19,25 +18,21 @@ pub trait ClipboardProvider {
 
 #[derive(MallocSizeOf)]
 pub(crate) struct EmbedderClipboardProvider {
-    pub constellation_sender: ScriptToConstellationChan,
+    pub embedder_sender: ScriptToEmbedderChan,
     pub webview_id: WebViewId,
 }
 
 impl ClipboardProvider for EmbedderClipboardProvider {
     fn get_text(&mut self) -> Result<String, String> {
         let (tx, rx) = channel().unwrap();
-        self.constellation_sender
-            .send(ScriptToConstellationMessage::ForwardToEmbedder(
-                EmbedderMsg::GetClipboardText(self.webview_id, tx),
-            ))
+        self.embedder_sender
+            .send(EmbedderMsg::GetClipboardText(self.webview_id, tx))
             .unwrap();
         rx.recv().unwrap()
     }
     fn set_text(&mut self, s: String) {
-        self.constellation_sender
-            .send(ScriptToConstellationMessage::ForwardToEmbedder(
-                EmbedderMsg::SetClipboardText(self.webview_id, s),
-            ))
+        self.embedder_sender
+            .send(EmbedderMsg::SetClipboardText(self.webview_id, s))
             .unwrap();
     }
 }