Delete the omahaproxy-backed extension blacklist and clear its entries from the
preferences file.

BUG=295882
R=shess@chromium.org, yoz@chromium.org

Review URL: https://codereview.chromium.org/23591050

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224680 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224680