Using mmap-allocated memory for backing stores does not allow us to
benefit from using GPU-accelerated painting, because all the performance
increase we get is mostly negated by reading the GPU-allocated texture
back into RAM, so it can be shared with the browser process.
With IOSurface, we get a framebuffer that is both shareable between
processes and can be used as underlying memory for an OpenGL/Metal
texture.
This change does not yet benefit from using IOSurface and merely wraps
them into Gfx::Bitmap to be used by the CPU painter.
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.
