Service processes (processes with --type=service) are child processes that are intended to possibly outlive the browser process that spawned them. They are currently used by the cloud print feature. Service processes can be set to start at login. They communicate with a browser via standard chromium IPC channels, but the way these channels is set up varies by platform.
Service processes are launched at login by adding them to the current users auto run key path in the registry.
Service processes are launched at login by adding an entry to the autostart file in $XDG_CONFIG_HOME.
Service processes are launched at login by adding a property list (.plist) to ~/Library/LaunchAgents. This means that service processes on MacOS X are controlled by launchd. This has some interesting ramifications that are discussed below.
Service processes continue to run as long as the service that they are providing is enabled and/or there is a browser process attached to them. Service processes are identified on the system by a unique identifier based on a service name, the user-data directory and the version of the service process supplying the service. If the user enables something that requires a service process (eg CloudPrint) the browser will launch the service process immediately, and set it up so that the service process will be started at login. On Windows and Linux, the service process is simply launched, while on Mac OS X the service process is started as a launchd job. Browser processes can force a service process to quit. On Windows and Linux this is performed by sending the equivalent of a SIGTERM to the service process. On MacOS X this is performed by telling launchd to remove the service process which in turn causes launchd to send the service process a SIGTERM. The relevant code is in ForceServiceProcessShutdown(...).
MacOS X note: Since service processes are launched by launchd on MacOS X they will automatically be relaunched if they ever crash. The proper way to stop the service processes manually on MacOS X is to manipulate them using launchctl. Sending a SIGTERM directly to the child process without using launchctl will result in a job still being registered with launchd. Launching a service process manually on MacOS X is difficult as it requires a properly configured property list containing several entries.
IPC is performed using a named pipe. The pipe name is based on the path to the user-data directory and the version of the browser/service creating the pipe.
IPC is performed using unix domain sockets. The name of the socket is based on the path to the user-data directory and the version of the browser/service creating the sockets.
IPC is performed using unix domain sockets. The name of the socket is assigned to the browser by looking up an environment variable that is set by launchd. On the service process side, the IPC channel is created using the file descriptor that is passed to the service process when it checks in with launchd. More information on this can be found on the launchd man page. The name of the environment variable is based on the path to the user-data directory and the version of the browser/service creating the sockets.