service worker: Simplify LazyInitializeForTest.

So callsites don't need to make a RunLoop.

Change-Id: I499166d235446cbe761ba045e2597f01d92abef8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1743238
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Auto-Submit: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685099}