Add first class support for user scripts.

Original review: http://codereview.chromium.org/340057

TBR=mpcomplete@chromium.org
BUG=22103
TEST=Install a user script (such as from userscripts.org). You should get the extension install UI and the
script should show up in the extension management UI. It should also work, though some scripts use
Firefox-specific APIs and those won't work in Chromium.

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@30925 0039d316-1c4b-4281-b951-d872f2087c98