Docserver: Enable GitHub content providers.

BUG=293678
R=jyasskin@chromium.org

Review URL: https://codereview.chromium.org/61393002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233413 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233413