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/constellation/from_script_message.rs

@@ -17,9 +17,9 @@ use canvas_traits::canvas::{CanvasId, CanvasMsg};
 use compositing_traits::CrossProcessCompositorApi;
 use devtools_traits::{DevtoolScriptControlMsg, ScriptToDevtoolsControlMsg, WorkerId};
 use embedder_traits::{
-    AnimationState, EmbedderMsg, FocusSequenceNumber, JSValue, JavaScriptEvaluationError,
-    JavaScriptEvaluationId, MediaSessionEvent, Theme, TouchEventResult, ViewportDetails,
-    WebDriverMessageId,
+    AnimationState, FocusSequenceNumber, JSValue, JavaScriptEvaluationError,
+    JavaScriptEvaluationId, MediaSessionEvent, ScriptToEmbedderChan, Theme, TouchEventResult,
+    ViewportDetails, WebDriverMessageId,
 };
 use euclid::default::Size2D as UntypedSize2D;
 use fonts_traits::SystemFontServiceProxySender;
@@ -444,6 +444,8 @@ pub struct WorkerGlobalScopeInit {
     pub from_devtools_sender: Option<IpcSender<DevtoolScriptControlMsg>>,
     /// Messages to send to constellation
     pub script_to_constellation_chan: ScriptToConstellationChan,
+    /// Messages to send to the Embedder
+    pub script_to_embedder_chan: ScriptToEmbedderChan,
     /// The worker id
     pub worker_id: WorkerId,
     /// The pipeline id
@@ -525,8 +527,6 @@ pub enum ScriptToConstellationMessage {
     /// Broadcast a message to all same-origin broadcast channels,
     /// excluding the source of the broadcast.
     ScheduleBroadcast(BroadcastChannelRouterId, BroadcastChannelMsg),
-    /// Forward a message to the embedder.
-    ForwardToEmbedder(EmbedderMsg),
     /// Broadcast a storage event to every same-origin pipeline.
     /// The strings are key, old value and new value.
     BroadcastStorageEvent(