Add base:: namespace to string16 in chrome/browser/extensions.

R=yoz@chromium.org
BUG=none
TEST=compile

Review URL: https://codereview.chromium.org/108813011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241260 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241260