Update comment about NaCl UMA. Histogram entries are public.

Moved to public repo at r252429.

BUG=none

Review URL: https://codereview.chromium.org/183293003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253897 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253897