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/pipeline.rs

@@ -27,7 +27,9 @@ use constellation_traits::{LoadData, SWManagerMsg, ScriptToConstellationChan};
 use crossbeam_channel::{Sender, unbounded};
 use devtools_traits::{DevtoolsControlMsg, ScriptToDevtoolsControlMsg};
 use embedder_traits::user_content_manager::UserContentManager;
-use embedder_traits::{AnimationState, FocusSequenceNumber, Theme, ViewportDetails};
+use embedder_traits::{
+    AnimationState, FocusSequenceNumber, ScriptToEmbedderChan, Theme, ViewportDetails,
+};
 use fonts::{SystemFontServiceProxy, SystemFontServiceProxySender};
 use ipc_channel::Error;
 use ipc_channel::ipc::{self, IpcReceiver, IpcSender};
@@ -131,6 +133,9 @@ pub struct InitialPipelineState {
     /// A channel to the associated constellation.
     pub script_to_constellation_chan: ScriptToConstellationChan,
 
+    /// A channel to the embedder.
+    pub script_to_embedder_chan: ScriptToEmbedderChan,
+
     /// A sender to request pipeline namespace ids.
     pub namespace_request_sender: GenericSender<PipelineNamespaceRequest>,
 
@@ -277,6 +282,7 @@ impl Pipeline {
                     parent_pipeline_id: state.parent_pipeline_id,
                     opener: state.opener,
                     script_to_constellation_chan: state.script_to_constellation_chan.clone(),
+                    script_to_embedder_chan: state.script_to_embedder_chan,
                     namespace_request_sender: state.namespace_request_sender,
                     background_hang_monitor_to_constellation_chan: state
                         .background_hang_monitor_to_constellation_chan
@@ -476,6 +482,7 @@ pub struct UnprivilegedPipelineContent {
     opener: Option<BrowsingContextId>,
     namespace_request_sender: GenericSender<PipelineNamespaceRequest>,
     script_to_constellation_chan: ScriptToConstellationChan,
+    script_to_embedder_chan: ScriptToEmbedderChan,
     background_hang_monitor_to_constellation_chan: IpcSender<HangMonitorAlert>,
     bhm_control_port: Option<IpcReceiver<BackgroundHangMonitorControlMsg>>,
     devtools_ipc_sender: Option<IpcSender<ScriptToDevtoolsControlMsg>>,
@@ -530,6 +537,7 @@ impl UnprivilegedPipelineContent {
                 constellation_sender: self.script_chan.clone(),
                 constellation_receiver: self.script_port,
                 pipeline_to_constellation_sender: self.script_to_constellation_chan.clone(),
+                pipeline_to_embedder_sender: self.script_to_embedder_chan,
                 background_hang_monitor_register: background_hang_monitor_register.clone(),
                 #[cfg(feature = "bluetooth")]
                 bluetooth_sender: self.bluetooth_thread,