Rename tracing_untrusted->tracing_nacl

BUG=345453

Review URL: https://codereview.chromium.org/185063002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254628 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254628