service worker: Fix UI thread registration knowledge on startup.

ServiceWorkeContextWatcher::Start was called too early, resulting
in trying to read all registrations before the context core was
initialized, which simply aborts silently, so the UI thread never got
the list of existing registrations upon startup.

Change-Id: I694ee9375cb169746fa6d362444c276817e26606
Bug: 991143
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1739229
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684711}