Accept L2TP Xauth parameters in ONC

Add Xauth L2TP credentials to the ONC configuration language.

BUG=267647
R=pneubeck@chromium.org

Review URL: https://codereview.chromium.org/141583002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246079 0039d316-1c4b-4281-b951-d872f2087c98
Change extension-N crash keys to be 1-indexed, not 0-indexed.

BUG=336435
TEST=Covered by unit tests.

Review URL: https://codereview.chromium.org/144013006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246080 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246080