autotestPrivate.lockScreen implementation.

BUG=233834
TEST=manual

Review URL: https://chromiumcodereview.appspot.com/24276002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224684 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224684