Add policies to control power management on the Chrome OS login screen
This CL adds a device policy that controls power management on the login
screen. Before this CL, powerd controlled power policy on the login screen,
always using built-in defaults. After this CL, Chrome contols power policy
by explicitly sending it to powerd whenever the login screen is shown. The
built-in defaults match those that powerd used to have but they can now be
overridden through device policy.
BUG=241794
TEST=Full coverage with new unit and broweser tests
Review URL: https://chromiumcodereview.appspot.com/
18153007
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@212024 0039d316-1c4b-4281-b951-d872f2087c98