Bug 732553 - NS_BASE_STREAM_CLOSED in dbg-transport.js
Cherry pick, rebased, Bug 878958 - attachment 760057: Support connections with prefixed actor names, and prefix-based forwarding.
Cherry pick, as-is, Bug 878958 - attachment 760055: Implement ChildDebuggerTransport, a debug protocol transport for communicating with content child processes via process message managers.
Bug 817580 - Expose tab actors for apps in child processes.