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>

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