NaCl: Clean up how FDs are passed to nacl_helper instances on Linux

child_process_launcher.cc constructs a mapping from FD numbers to FDs,
and normal Chromium child processes receive this as a mapping.
However, before this change, when zygote_linux.cc passed these FDs to
nacl_helper, it stripped the keys from the mapping and only passed the
values.

This meant that if child_process_launcher.cc were changed to add more
key+value pairs to the FD mapping, nacl_helper_linux.cc would need to
be updated to use or skip over them.  Also, the order in which
child_process_launcher.cc adds the FDs to the mapping would be
significant, but only to nacl_helper, which would be weird.  This
would be particularly awkward for FDs that are added to the mapping
conditionally.

We clean this up by explicitly taking the one FD that nacl_helper
needs from the FD mapping, rather than passing across all of the
mapping's values.

The aim of this cleanup is to simplify this change --
https://codereview.chromium.org/22911027/ -- which changes
child_process_launcher.cc to add an FD conditionally.

Also make the #defines of the FD numbers clearer by moving them to
zygote_fork_delegate_linux.h, since that's the component that's
responsible for them.

BUG=none
TEST=NaCl tests in browser_tests

Review URL: https://codereview.chromium.org/24449002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228228 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228228