Update-URL support for external extensions

This allows external extensions configured through registry keys to
use an update_url key to specify an update URL, rather than
restricting them to local CRX files only.

BUG=327137

Review URL: https://codereview.chromium.org/115543003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240986 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240986