Move xdg_util into base/nix. I originally wanted to use base/linux, but you
can't have a namespace called "linux" because it's ifdefed by GCC to be a
number. Same for "unix". "x11" was discarded because some future stuff in
here will not be related to x11.

This also renames base_paths_posix to base_paths_linux since this does not
apply to mac, and normally posix applies to mac (this confused me when I had to
edit it for the XDG stuff).

TEST=it compiles
BUG=none
Review URL: http://codereview.chromium.org/3778007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@62848 0039d316-1c4b-4281-b951-d872f2087c98
15 files changed