Add additional restriction to policy schema internal, including modification to
C++ verifier.

BUG=258339

Review URL: https://codereview.chromium.org/143413002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246328 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246328