In order to bring up services within a minimal runtime environment, and in order to avoid subtle lifetime dependency issues between service processes and the central Mojo broker process, the Service Manager should be a long-running standalone process which also serves as the broker.

This has immediately useful applications on both Windows (e.g. lightweight notification listener) and Chrome OS (many system services interfacing with containers/VMs), and most likely other platforms.

Brief breakdown of the work blocking (or semi-blocking) this

- moving process management code into the Service Manager (bug 689159, bug 781334)
- teasing out content dependency injection which we rely on today, i.e. most of [1] The specific motivation here is just a lightweight browser process, but it solves the same problem (bug 729596)
- supporting external binaries so e.g. system services may be controlled by SM (bug 809320)
- (Chrome OS) figuring out how to get Service Manager to be trusted enough to
  spawn system service processes

[1] https://cs.chromium.org/chromium/src/content/browser/service_manager/service_manager_context.cc