Service Worker: Remove legacy code for resolving register() to a version

Service Worker: Remove legacy code for resolving register() to a version

register() resolves to a ServiceWorkerRegistration, so we no longer need
to pass a ServiceWorkerVersion around.

This also cleans up a couple TODOs in RegisterJob:
- In ContinueWithRegistrationForSameScriptUrl, the TODO made sense when
register resolved to a version not a registration.
Now, the current behavior seems OK.
- In UpdateAndContinue, the TODO was misleading. We don't have to handle
an existing installing worker within RegisterJob since each job either
succeeds with a waiting/active worker or fails and cleans up the
installing worker.

BUG=406240

Review URL: https://codereview.chromium.org/605163002

Cr-Commit-Position: refs/heads/master@{#297131}