Track the active tab and browsing context for devtools (#36168)

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---Part of #35867, per Step 5 suggestion. This PR:
- Adds active_tab (via RefCell) to RootActor, updated in
get_tab_msg_by_browser_id.
- Adds browsing_context() helper to TabDescriptorActor.
- Adds actor()`getter to TabDescriptorActorMsg for  access in root.rs.
-Creates the chain (Root - Tab - BrowsingContext) for routing
colorSchemeSimulation.
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- [X ] `./mach build -d` does not report any errors
- [X ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because they’re structural
setup for the actor chain and don’t change the system's behavior yet.

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---------

Signed-off-by: Uthman Yahaya Baba <uthmanyahayababa@gmail.com>

components/devtools/actors/root.rs

@@ -9,6 +9,7 @@
 //!
 //! [Firefox JS implementation]: https://searchfox.org/mozilla-central/source/devtools/server/actors/root.js
 
+use std::cell::RefCell;
 use std::net::TcpStream;
 
 use serde::Serialize;
@@ -129,6 +130,7 @@ pub struct RootActor {
     pub device: String,
     pub preference: String,
     pub process: String,
+    pub active_tab: RefCell<Option<String>>,
 }
 
 impl Actor for RootActor {
@@ -303,13 +305,24 @@ impl RootActor {
         registry: &ActorRegistry,
         browser_id: u32,
     ) -> Option<TabDescriptorActorMsg> {
-        self.tabs
+        let tab_msg = self
+            .tabs
             .iter()
             .map(|target| {
                 registry
                     .find::<TabDescriptorActor>(target)
                     .encodable(registry, true)
             })
-            .find(|tab| tab.browser_id() == browser_id)
+            .find(|tab| tab.browser_id() == browser_id);
+
+        if let Some(ref msg) = tab_msg {
+            *self.active_tab.borrow_mut() = Some(msg.actor());
+        }
+        tab_msg
+    }
+
+    #[allow(dead_code)]
+    pub fn active_tab(&self) -> Option<String> {
+        self.active_tab.borrow().clone()
     }
 }

components/devtools/actors/tab.rs

@@ -43,6 +43,10 @@ impl TabDescriptorActorMsg {
     pub fn browser_id(&self) -> u32 {
         self.browser_id
     }
+
+    pub fn actor(&self) -> String {
+        self.actor.clone()
+    }
 }
 
 #[derive(Serialize)]
@@ -167,4 +171,9 @@ impl TabDescriptorActor {
     pub(crate) fn is_top_level_global(&self) -> bool {
         self.is_top_level_global
     }
+
+    #[allow(dead_code)]
+    pub fn browsing_context(&self) -> String {
+        self.browsing_context_actor.clone()
+    }
 }

components/devtools/lib.rs

@@ -153,6 +153,7 @@ impl DevtoolsInstance {
             performance: performance.name(),
             preference: preference.name(),
             process: process.name(),
+            active_tab: None.into(),
         });
 
         registry.register(root);