Add base:: to a string16 in printing/.

R=dpapad@chromium.org
BUG=329295

Review URL: https://codereview.chromium.org/101643006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241974 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241974