The previous approach of killing the Chromium parent process sometimes caused leaks of child processes on Windows. The Browser.close command in the debug protocol will tell Chromium to shut down all of its processes.
This implements https://github.com/flutter/flutter/issues/132654#issuecomment-1738221257, namely:
Make `Chromium.close` more robust:
* Send `SIGTERM` and wait up to 5 seconds, if the process exited, great! Return from the function.
* If the process has not exited, then send a `SIGKILL`, which is a much firmer way to exit a process. Same as before, wait up to 5 seconds, if the process exited, great! Return from the function.
* If it still hasn't exited then give up trying to exit Chromium, just print a warning to the console and return from the function.
Bonus: a few nullability fixes and extra `-v` logging.
Fixes https://github.com/flutter/flutter/issues/132654
