Pepper: Allow RequestOSFileDescriptor for PRIVATE.

The FileIO_Private API contains the RequestOSFileDescriptor call. However, the
host currently only allows it to be used by whitelisted extensions or if a flag
is passed.

This change also permits the call for plugins with PERMISSION_PRIVATE. This
allows this interface to be used by the NaCl "trusted plugin".

BUG=246396
R=bbudge@chromium.org

Review URL: https://codereview.chromium.org/25049005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226024 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226024