Fix some string16 -> base::string16 that have crept into ui/.

TBR=sky@chromium.org
BUG=329295

Review URL: https://codereview.chromium.org/113203004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242096 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242096