constellation: Only return focused browsing contexts that exist. (#36330)

The webdriver server relies on the constellation to report which
browsing context is focused, and assumes that a focused context is ready
for interaction. However, new browsing contexts exist in a weird state
where they are not tracked by the constellation until the initial load
is complete, which leads to the constellation rejecting attempts to
navigate a browsing context right after it's created. These changes
ensure the constellation does not report a browsing context as focused
until it's actually created and ready for interaction.

Testing: Run `./mach test-wpt --product servodriver
tests/wpt/mozilla/tests/mozilla/DOMParser.html`, which now runs to
completion.
Fixes: #34551
Fixes: #36328

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

tests/wpt/meta/MANIFEST.json

@@ -510216,7 +510216,7 @@
         []
        ],
        "servodriver.py": [
-        "bd16d7987e539a3325d88876982462ea1a3eb638",
+        "7121e1dafcc2d55ebc18bdf3e922ce9978b94ee1",
         []
        ],
        "webkit.py": [

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

@@ -101,7 +101,7 @@ class ServoWebDriverBrowser(WebDriverBrowser):
             # For some reason rustls does not like the certificate generated by the WPT tooling.
             "--ignore-certificate-errors",
             "--window-size", "800x600",
-            "data:,",
+            "about:blank",
         ]
 
         ca_cert_path = server_config.ssl_config["ca_cert_path"]