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/shared/devtools/lib.rs

@@ -32,6 +32,7 @@ use uuid::Uuid;
 pub struct DevtoolsPageInfo {
     pub title: String,
     pub url: ServoUrl,
+    pub is_top_level_global: bool,
 }
 
 #[derive(Clone, Debug, Deserialize, MallocSizeOf, Serialize)]