Hide downloads of extensions blacklist from web request API.

      This CL prevents that extensions using the web request API can prevent Chrome from updating its extensions blacklist.

R=mpcomplete@chromium.org
BUG=108648
TEST=no


Review URL: http://codereview.chromium.org/8952021

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@116258 0039d316-1c4b-4281-b951-d872f2087c98