Allow screenlockPrivate.showButton to be called with different icons.

TEST=manaul

Review URL: https://codereview.chromium.org/137233009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246443 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246443