devtools: Box the actor inside register (#41074)

Small refactor to reduce verbosity when creating actors. Since they
always need to be boxed, do that in `register{_later}` instead. I also
added the `Send` trait to `Actor` directly since it is always needed.

Testing: This patch doesn't change behaviour.

Signed-off-by: eri <eri@igalia.com>

components/devtools/actors/browsing_context.rs

@@ -273,14 +273,14 @@ impl BrowsingContextActor {
             watcher: watcher.name(),
         };
 
-        actors.register(Box::new(accessibility));
-        actors.register(Box::new(css_properties));
-        actors.register(Box::new(inspector));
-        actors.register(Box::new(reflow));
-        actors.register(Box::new(style_sheets));
-        actors.register(Box::new(tabdesc));
-        actors.register(Box::new(thread));
-        actors.register(Box::new(watcher));
+        actors.register(accessibility);
+        actors.register(css_properties);
+        actors.register(inspector);
+        actors.register(reflow);
+        actors.register(style_sheets);
+        actors.register(tabdesc);
+        actors.register(thread);
+        actors.register(watcher);
 
         target
     }