Add management policy function allowing extensions to be disabled.

This CL adds the MustRemainDisabled function to the ManagementPolicy::Provider
interface and calls it in appropriate places.

BUG=314276

Review URL: https://codereview.chromium.org/54903011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232976 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232976
Make an include path relative to src/

NOTRY=true

Review URL: https://codereview.chromium.org/54983009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232977 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232977