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>

components/constellation/constellation.rs

@@ -4422,6 +4422,7 @@ where
             WebDriverCommandMsg::GetWindowRect(..) |
             WebDriverCommandMsg::GetViewportSize(..) |
             WebDriverCommandMsg::SetWindowRect(..) |
+            WebDriverCommandMsg::MaximizeWebView(..) |
             WebDriverCommandMsg::LoadUrl(..) |
             WebDriverCommandMsg::Refresh(..) |
             WebDriverCommandMsg::SendKeys(..) |