Enable experimental features in servodriver. (#38648)

Applies the change from https://github.com/servo/servo/pull/36335 to the
non-default servodriver WPT executor as well.

Testing: Tested manually with `--product=servodriver` on the webxr
directory.
Part of #34683

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

tests/wpt/meta/MANIFEST.json

@@ -522005,7 +522005,7 @@
         []
        ],
        "servodriver.py": [
-        "f72f2257a3817e33817dc7a190da8e61cf6442da",
+        "c25aa9b3ecdcfe20c6ff0e8f98973d69c86d5209",
         []
        ],
        "webkit.py": [

tests/wpt/tests/tools/wptrunner/wptrunner/browsers/servodriver.py

@@ -97,6 +97,7 @@ class ServoWebDriverBrowser(WebDriverBrowser):
             # See https://github.com/servo/servo/issues/30080.
             # For some reason rustls does not like the certificate generated by the WPT tooling.
             "--ignore-certificate-errors",
+            "--enable-experimental-web-platform-features",
             "--window-size", "800x600",
             "about:blank",
         ]