Add WebServiceWorkerErrorType::ErrorTypeAbort

This is needed for removing --enable-service-worker
(https://codereview.chromium.org/339973003/).  Chromium-side currently
returns ErrorTypeDisabled for both "SW is disabled" and "SWContextCore
was destroyed during shutdown" when handling a registration request.
Since the former will no longer happen, add a generic operation aborted
error for the latter.

BUG=365201

Review URL: https://codereview.chromium.org/342863003

git-svn-id: svn://svn.chromium.org/blink/trunk@176500 bbb929c8-8fbe-4397-9dbb-9b2b20218538