Revert 226494 "Allow explicitly whitelisted apps/extensions in p..."

Most likely cause of test failures at

http://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%28dbg%29%282%29/builds/9496

Failed tests:
  BrowserGuestSessionNavigatorTest.Disposition_Settings_UseIncognitoWindow
  UserTypeInstantiation/AccessibilityManagerUserTypeTest.EnableOnLoginScreenAndLogin/1

> Allow explicitly whitelisted apps/extensions in public sessions
> 
> This CL adds an extension management policy provider that allows
> explicitly whitelisted apps/extensions to be installed in public sessions.
> Right now, QuickOffice and all hosted apps are whitelisted.
> 
> BUG=296868
> TEST=New browser and unit tests
> 
> Review URL: https://codereview.chromium.org/24261010

TBR=bartfab@chromium.org

Review URL: https://codereview.chromium.org/25757003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226635 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226635