ASSERTION FAILED: registration in WebCore::SWServerWorker::skipWaiting()

ASSERTION FAILED: registration in WebCore::SWServerWorker::skipWaiting()
https://bugs.webkit.org/show_bug.cgi?id=181222
<rdar://problem/36332686>

Reviewed by Youenn Fablet.

Source/WebCore:

Replace assertion in SWServerWorker::skipWaiting() that assumes the worker
has a registration. Nowadays, a SWServerWorker can stay alive for a short
period without having a registration, while it is terminating.

No new tests, unskipped existing test.

* workers/service/server/SWServerWorker.cpp:
(WebCore::SWServerWorker::skipWaiting):

LayoutTests:

Unskip test that is no longer flakily crashing.

* TestExpectations:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@226813 268f45cc-cd09-0410-ab3c-d52691b4dbfc