We want to use POSIX shared memory to share the field trial allocator on MacOS, but it's not available. We need to do some work to be able to use it.

1) Create SharedMemoryPosix
2) On non-MacOS POSIX systems, get SharedMemory to subclass from SharedMemoryPosix so as to not regress any current behavior
3) In shared_memory_mac.cc give the SharedMemory class a std::unique_ptr<SharedMemoryPosix>