ServiceWorker: Remove unnecessary references to ServiceWorkerVersion
ServiceWorkerRegistrationHandle doesn't have to own references to the
associated workers because |registration_| owns them.
SetVersionAttributes() via OnRegistrafionFailed() might send an invalid
version info for the previously cleared attribute, but that's harmless
and just clears it again.
BUG=n/a
TEST=should pass all existing tests
Review URL: https://codereview.chromium.org/
850273002
Cr-Commit-Position: refs/heads/master@{#311852}