webdriver: Improve parsing of Frame and Window (#39012)

In #38745, we changed the id of Frame and Window as the result of
`ToString` trait. This PR
- adapts the parsing of frame/window accordingly.
- for frame, return the
[WindowProxy](https://developer.mozilla.org/en-US/docs/Web/API/WindowProxy)
object of the iframe as it's supposed to do.

Testing: `execute_{async_}script/arguments.py`

---------

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

components/script_bindings/webidls/Window.webidl

@@ -151,7 +151,7 @@ partial interface Window {
   undefined webdriverException(optional any result);
   undefined webdriverTimeout();
   Element? webdriverElement(DOMString id);
-  Element? webdriverFrame(DOMString id);
+  WindowProxy? webdriverFrame(DOMString id);
   WindowProxy? webdriverWindow(DOMString id);
   ShadowRoot? webdriverShadowRoot(DOMString id);
 };