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/constellation/constellation.rs

@@ -134,8 +134,8 @@ use embedder_traits::{
     AnimationState, CompositorHitTestResult, EmbedderMsg, EmbedderProxy, FocusId,
     FocusSequenceNumber, InputEvent, JSValue, JavaScriptEvaluationError, JavaScriptEvaluationId,
     KeyboardEvent, MediaSessionActionType, MediaSessionEvent, MediaSessionPlaybackState,
-    MouseButton, MouseButtonAction, MouseButtonEvent, Theme, ViewportDetails, WebDriverCommandMsg,
-    WebDriverCommandResponse, WebDriverLoadStatus, WebDriverScriptCommand,
+    MouseButton, MouseButtonAction, MouseButtonEvent, ScriptToEmbedderChan, Theme, ViewportDetails,
+    WebDriverCommandMsg, WebDriverCommandResponse, WebDriverLoadStatus, WebDriverScriptCommand,
 };
 use euclid::Size2D;
 use euclid::default::Size2D as UntypedSize2D;
@@ -968,6 +968,10 @@ where
             self.public_resource_threads.clone()
         };
 
+        let embedder_chan = self.embedder_proxy.sender.clone();
+        let eventloop_waker = self.embedder_proxy.event_loop_waker.clone();
+        let script_to_embedder_chan = ScriptToEmbedderChan::new(embedder_chan, eventloop_waker);
+
         let result = Pipeline::spawn::<STF>(InitialPipelineState {
             id: pipeline_id,
             browsing_context_id,
@@ -978,6 +982,7 @@ where
                 sender: self.script_sender.clone(),
                 pipeline_id,
             },
+            script_to_embedder_chan,
             namespace_request_sender: self.namespace_ipc_sender.clone(),
             pipeline_namespace_id: self.next_pipeline_namespace_id(),
             background_monitor_register: self.background_monitor_register.clone(),
@@ -1702,9 +1707,6 @@ where
                 self.broadcast_channels
                     .schedule_broadcast(router_id, message);
             },
-            ScriptToConstellationMessage::ForwardToEmbedder(embedder_msg) => {
-                self.embedder_proxy.send(embedder_msg);
-            },
             ScriptToConstellationMessage::PipelineExited => {
                 self.handle_pipeline_exited(source_pipeline_id);
             },