constellation: Proxy ScrollState through the Constellation (#36062)

This will allow removing the dependency of the compositor on
`script_traits`, which should make our internal dependency chain a lot
easier to deal with.

Part of #35984.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/constellation/constellation.rs

@@ -160,7 +160,7 @@ use webgpu::{self, WebGPU, WebGPURequest, WebGPUResponse};
 use webrender::RenderApi;
 use webrender::RenderApiSender;
 use webrender_api::{DocumentId, ImageKey};
-use webrender_traits::{CompositorHitTestResult, WebrenderExternalImageRegistry};
+use webrender_traits::{CompositorHitTestResult, ScrollState, WebrenderExternalImageRegistry};
 
 use crate::browsingcontext::{
     AllBrowsingContextsIterator, BrowsingContext, FullyActiveBrowsingContextsIterator,
@@ -1406,6 +1406,9 @@ where
             FromCompositorMsg::SetWebViewThrottled(webview_id, throttled) => {
                 self.set_webview_throttled(webview_id, throttled);
             },
+            FromCompositorMsg::SetScrollStates(pipeline_id, scroll_states) => {
+                self.handle_set_scroll_states(pipeline_id, scroll_states)
+            },
         }
     }
 
@@ -5562,4 +5565,24 @@ where
             error!("Got a media session action but no active media session is registered");
         }
     }
+
+    #[cfg_attr(
+        feature = "tracing",
+        tracing::instrument(skip_all, fields(servo_profiling = true), level = "trace")
+    )]
+    fn handle_set_scroll_states(&self, pipeline_id: PipelineId, scroll_states: Vec<ScrollState>) {
+        let Some(pipeline) = self.pipelines.get(&pipeline_id) else {
+            warn!("Discarding scroll offset update for unknown pipeline");
+            return;
+        };
+        if let Err(error) = pipeline
+            .event_loop
+            .send(ScriptThreadMessage::SetScrollStates(
+                pipeline_id,
+                scroll_states,
+            ))
+        {
+            warn!("Could not send scroll offsets to pipeline: {pipeline_id:?}: {error:?}");
+        }
+    }
 }

components/constellation/tracing.rs

@@ -86,6 +86,7 @@ mod from_compositor {
                 Self::ExitFullScreen(_) => target!("ExitFullScreen"),
                 Self::MediaSessionAction(_) => target!("MediaSessionAction"),
                 Self::SetWebViewThrottled(_, _) => target!("SetWebViewThrottled"),
+                Self::SetScrollStates(..) => target!("SetScrollStates"),
             }
         }
     }