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This CL is to build base/ for nacl_helper_nonsfi.
The library is similar to base_nacl, but slightly different:
- For rand_util, rand_util_posix should be used, instead of
  rand_util_nacl, because nacl_helper_nonsfi will be running under
  Linux directly.
- MessageLoopForIO should be based on MessagePumpLibevent rather than
  MessagePumpDefault, to support IPC.
- GetKnownDeadTerminationStatus, GetTerminationStatus,
  UnixDomainSocket::SendMsg and RecvMsg are included, as these are used
  to implement nacl_helper_nonsfi binary.
- GLIB is not supported. It is unnecessary for nacl_helper_nonsfi.
Note that this library is not used yet from any binary, because this CL
is just a preparation, but the library is built actually.

BUG=358465
TEST=Ran trybot. Implement nacl_helper_nonsfi on top of this CL, and made sure
it is working.

Review URL: https://codereview.chromium.org/659513004

Cr-Commit-Position: refs/heads/master@{#300075}