Register iframes with the devtools (#35874)

Previously, the devtools didn't know about
<iframe>s. They either ignored messages coming from
iframes or crashed.

This reverts https://github.com/servo/servo/pull/34032
and then filters out non-tab globals in the "listTabs"
message to the root actor.

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

components/devtools/actors/root.rs

@@ -200,10 +200,14 @@ impl Actor for RootActor {
                     tabs: self
                         .tabs
                         .iter()
-                        .map(|target| {
-                            registry
-                                .find::<TabDescriptorActor>(target)
-                                .encodable(registry, false)
+                        .filter_map(|target| {
+                            let tab_actor = registry.find::<TabDescriptorActor>(target);
+                            // Filter out iframes and workers
+                            if tab_actor.is_top_level_global() {
+                                Some(tab_actor.encodable(registry, false))
+                            } else {
+                                None
+                            }
                         })
                         .collect(),
                 };