Auto merge of #21943 - codehag:devtools-add-list-workers-method, r=jdm

DevTools - add ListWorkers method to BrowsingContextTargetActor

This is one of three pull requests that allows the DevTools Debugger to render.

The two related prs are #21942 and #21944

This pr introduces a `ListWorkers` method, which the debugger relies on for startup. At the moment, we are returning an empty array, but later on we can return an array populated by all workers associated with a target.

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components/devtools/actors/browsing_context.rs

@@ -54,6 +54,17 @@ struct FrameMsg {
     parentID: u32,
 }
 
+#[derive(Serialize)]
+struct ListWorkersReply {
+    from: String,
+    workers: Vec<WorkerMsg>,
+}
+
+#[derive(Serialize)]
+struct WorkerMsg {
+    id: u32,
+}
+
 #[derive(Serialize)]
 pub struct BrowsingContextActorMsg {
     actor: String,
@@ -159,6 +170,15 @@ impl Actor for BrowsingContextActor {
                 ActorMessageStatus::Processed
             },
 
+            "listWorkers" => {
+                let msg = ListWorkersReply {
+                    from: self.name(),
+                    workers: vec![],
+                };
+                stream.write_json_packet(&msg);
+                ActorMessageStatus::Processed
+            },
+
             _ => ActorMessageStatus::Ignored,
         })
     }