This large block of code is repeated nearly verbatim in LibWeb. Move it
to a helper function that both LibIPC and LibWeb can defer to. This will
let us make changes to this method in a singular location going forward.
Note this is a bit of a regression for the MessagePort. It now suffers
from the same performance issue that IPC messages face - we prepend the
meessage size to the message buffer. This degredation is very temporary
though, as a fix is imminent, and this change makes that fix easier.
