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

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226494 0039d316-1c4b-4281-b951-d872f2087c98
Disable flake LauncherAppBrowserTest.RefocusFilter.

Observed during sheriff shift: http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20ChromeOS&testType=browser_tests&tests=LauncherAppBrowserTest.RefocusFilter

BUG=303231
TBR=stevenjb@chromium.org

Review URL: https://codereview.chromium.org/25650004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226495 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226495