wpt: Set servo window size and open blank page for wdspec (#38793)

On `ServoWdSpecBrowser`, we set the window size to `800x600` and open
`about:blank`. This will unify the behavior of servo and servodriver.


604b6ea26d/tests/wpt/tests/tools/wptrunner/wptrunner/browsers/servodriver.py (L101-L102)

Testing: Unify the behavior of servo and servodriver for wdspec

Signed-off-by: PotatoCP <Kenzie.Raditya.Tirtarahardja@huawei.com>

tests/wpt/meta/MANIFEST.json

@@ -522937,7 +522937,7 @@
         ]
        },
        "servo.py": [
-        "07441780ccc7682c6b7616271aeb9a40310a433a",
+        "2928ffb7f1d483cff60dd17ffd92a53c9b1c3148",
         []
        ],
        "servodriver.py": [

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

@@ -116,7 +116,10 @@ class ServoWdspecBrowser(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",
-                   "--headless"] + self.webdriver_args
+                   "--headless",
+                   "--window-size",
+                   "800x600",
+                   "about:blank"] + self.webdriver_args
         if self.binary_args:
             command += self.binary_args
         return command