Add policy for fullscreen mode; disallow fullscreen in public sessions

This CL adds a user policy that determines whether fullscreen mode is
allowed and uses it to disallow fullscreen mode in public sessions.

BUG=275405
TEST=New browser tests

Review URL: https://chromiumcodereview.appspot.com/22986010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@219482 0039d316-1c4b-4281-b951-d872f2087c98
17 files changed