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/shared/embedder/webdriver.rs

@@ -138,6 +138,8 @@ pub enum WebDriverCommandMsg {
         DeviceIndependentIntRect,
         IpcSender<DeviceIndependentIntRect>,
     ),
+    /// Maximize the window. Send back result window rectangle.
+    MaximizeWebView(WebViewId, IpcSender<DeviceIndependentIntRect>),
     /// Take a screenshot of the viewport.
     TakeScreenshot(
         WebViewId,