This uses BrowserEngineKit's ExtensionKit-based processes to start content
processes on iOS. These processes are started with an initial xpc connection,
which is then used to communicate a command line, initial file descriptors
and environment variables before invoking content_process_main.
The XPC connection is not used further after the bootstrap message, which seems
to roughly match how WebKit uses these APIs.
Differential Revision: https://phabricator.services.mozilla.com/D202525
Nika Layzell