chromeos: Rename kEnableScreenLock to kEnableAutoScreenLock.

Rename a confusingly-named pref.

BUG=none
TBR=rsimha@chromium.org

Review URL: https://codereview.chromium.org/143823005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246636 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246636