webdriver: Implement maximize window for both headless&headed window (#38271)

- Implement [Maximize
Window](https://w3c.github.io/webdriver/#maximize-window)
- Previously, headless window screen size is same as inner size if not
specified in preference. We make it double as required by the test to
not have max window initially.
- Some other random cleanup.

Testing: webdriver Stress test for maximize window (headed + headless).

---------

Signed-off-by: Euclid Ye <euclid.ye@huawei.com>

ports/servoshell/desktop/window_trait.rs

@@ -65,4 +65,5 @@ pub trait WindowPortsMethods {
         servo::Theme::Light
     }
     fn window_rect(&self) -> DeviceIndependentIntRect;
+    fn maximize(&self, webview: &WebView);
 }