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/constellation/constellation.rs

@@ -4025,6 +4025,7 @@ where
                 let page_info = DevtoolsPageInfo {
                     title: new_pipeline.title.clone(),
                     url: new_pipeline.url.clone(),
+                    is_top_level_global: top_level_id == browsing_context_id,
                 };
                 let state = NavigationState::Stop(new_pipeline.id, page_info);
                 let _ = chan.send(DevtoolsControlMsg::FromScript(