Enforce the RemoteAccessHostRequireCurtain policy on all platforms.

The host now always creates the CurtainingHostObserver, and configures it
based upon policy.  The platform specific CurtainMode implementations then
handle attempts to activate the curtain according to availability; Windows
and Mac hosts will currently reject connections and Linux hosts will reject
connections if running against the console X server.

BUG=159711


Review URL: https://chromiumcodereview.appspot.com/11275178

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@167304 0039d316-1c4b-4281-b951-d872f2087c98
6 files changed