Various fixes for webdriver conformance tests (#35737)

* servodriver: Ensure capabilities is always a non-empty value.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* webdriver: Serialize arguments object like an array.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* webdriver: Ensure script body is always valid JS.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* webdriver: Use current browsing context when getting element center point.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* webdriver: Propagate errors received from getting element center point.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* webdriver: Ensure opening a new window records a unique window handle.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* webdriver: Don't panic if script execution fails.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* webdriver: Do not update the current browsing context after closing a window.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Formatting.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* webdriver: Use more precise check for arguments exotic object.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Formatting.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

tests/wpt/meta/MANIFEST.json

@@ -507579,7 +507579,7 @@
         []
        ],
        "servodriver.py": [
-        "18035f9331e0d5df1c3fa916ce33a8866bea091a",
+        "bd16d7987e539a3325d88876982462ea1a3eb638",
         []
        ],
        "webkit.py": [
@@ -507645,7 +507645,7 @@
         []
        ],
        "executorservodriver.py": [
-        "7e4aee5e7af7b9719b4ccb1f5a0a7c7cb3aa1653",
+        "e27b111ee8b7c4cdff4a0aee5c010d402efff263",
         []
        ],
        "executorwebdriver.py": [