This needs more unit tests for the various pieces of what's going on
here (LinuxCapabilities, SandboxChroot, UnshareUserNamespace()) but
that's nontrivial due to needing a single-threaded process -- and
currently they can't be run on Mozilla's CI anyway due to needing user
namespaces, and local testing can just try using GMP and manually
inspecting the child process. So that will be a followup.
This means that B2G plugin-container must (dynamically) link against
libmozsandbox in order to call into it before initializing Binder.
(Desktop Linux plugin-container already contains the sandbox code.)