Don't leak a WebServiceWorker when 'current' races with context destruction.

WebServiceWorkerProviderClient::setCurrentServiceWorker passes the
ownership of a WebServiceWorker to the container. If the execution
context is gone, delete the WebServiceWorker instead of dropping it on
the floor.

BUG=363967

Review URL: https://codereview.chromium.org/304023002

git-svn-id: svn://svn.chromium.org/blink/trunk@175023 bbb929c8-8fbe-4397-9dbb-9b2b20218538