Make UnsafePersistent::deprecatedHandle private, control access with friends.

The only users are ScriptWrappable, DOMDataStore and DOMWrapperMap. Ideally we'd
do the unsafe operations inside UnsafePersistent, but it's currently not
possible.

BUG=236290

Review URL: https://chromiumcodereview.appspot.com/22887031

git-svn-id: svn://svn.chromium.org/blink/trunk@156316 bbb929c8-8fbe-4397-9dbb-9b2b20218538