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 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/lib.rs

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