Identify and repair ONC files with duplicate GUIDs.

The patch identifies network configurations and certificates with duplicate GUIDs and repairs the ONC such that the first occurrence is retained as a unique network configuration or certificate.

BUG=211421

Review URL: https://codereview.chromium.org/166903002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251748 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251748