Docserver: Disable all github content providers. They haven't been working for
a while and may be messing up data elsewhere.

R=mkearney@chromium.org

Review URL: https://codereview.chromium.org/138443007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246409 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246409