Rename base_untrusted to base_nacl

BUG=345453
R=brettw@chromium.org
TBR=ben@chromium.org

Review URL: https://codereview.chromium.org/174493002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252732 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252732