Fix an assertion in net/url_request/url_request.cc:656

Instead of cancelling the request we have to set error to
net::ERR_ABORTED since URLRequest::BeforeRequestComplete
enforces not receiving cancelled requests with an assertion.
Our API requires the symmetric LoadStarten - LoadStopped
signals even on an ignored request so cancelling the request
and returning early would not be a solution, since it would
not emit any LoadStopped signal.

Change-Id: I1b13fe6520121805bcf420e810999c520c12bd5c
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>
Reviewed-by: Michael Bruning <michael.bruning@digia.com>