This large commit also refactors LibWebView's process handling to use
a top-level Application class that uses a new WebView::Process class to
encapsulate the IPC-centric nature of each helper process.
This will be needed to collect statistics from processes that do not
have anything to do with LibWebView. The ProcessInfo structure must be
virtual to allow callers to add application-specific information.
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.
