Add device policies to control accessibility settings on the login screen

This CL adds device policies that provide control over the default state
of the following four accessibility settings on the login screen:
* Large cursor
* Spoken feedback
* High contrast mode
* Screen magnifier type

The User can temporarily override the settings but the defaults are
restored whenever the login screen is shown anew or the user remains idle
on the login screen for one minute.

BUG=225955, 225956, 243350, 247298
TEST=New unittests + new browsertests + manual in VM

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=207755

Review URL: https://chromiumcodereview.appspot.com/16658015

git-svn-id: http://src.chromium.org/svn/trunk/src/chrome/browser/policy/proto@207889 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
1 file changed
tree: 526a738d75c9a1d8c3050cf5a9b127f2106ec18b
  1. chromeos/
  2. cloud/