chromeos: Send wait-for-user-activity pref to powerd.

Make the value of the "wait for initial user activity" pref
get included in PowerManagementPolicy protocol buffers sent
to the power manager.

BUG=310176

Review URL: https://codereview.chromium.org/46523003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231367 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231367