Improve inter-document focus handling (#36649)

*Describe the changes that this pull request makes here. This will be
the commit message.*
rewritten the PR #28571
Implement
[Window#focus](https://html.spec.whatwg.org/multipage/#dom-window-focus),
[Window#blur](https://html.spec.whatwg.org/multipage/#dom-window-blur)
Testing: WPT
Fixes: #8981 #9421

---------

Signed-off-by: kongbai1996 <1782765876@qq.com>
Co-authored-by: yvt <i@yvt.jp>

components/script/dom/windowproxy.rs

@@ -620,6 +620,23 @@ impl WindowProxy {
         result
     }
 
+    /// Run [the focusing steps] with this browsing context.
+    ///
+    /// [the focusing steps]: https://html.spec.whatwg.org/multipage/#focusing-steps
+    pub fn focus(&self) {
+        debug!(
+            "Requesting the constellation to initiate a focus operation for \
+            browsing context {}",
+            self.browsing_context_id()
+        );
+        self.global()
+            .script_to_constellation_chan()
+            .send(ScriptToConstellationMessage::FocusRemoteDocument(
+                self.browsing_context_id(),
+            ))
+            .unwrap();
+    }
+
     #[allow(unsafe_code)]
     /// Change the Window that this WindowProxy resolves to.
     // TODO: support setting the window proxy to a dummy value,