On macOS, it's not trivial to get a Mach task port for your children.
This implementation registers the chrome process as a well-known
service with launchd based on its pid, and lets each child process
send over a reference to its mach_task_self() back to the chrome.
We'll need this Mach task port right to get process statistics.
This implementation uses a really basic WebView to update stats once
a second. In the future it might make more sense to both move the
details into LibWebView, and to create a native widget for each platform
to remove the overhead of having an extra WebView.
