Prevent fetcher from being started without being stopped.

Change-Id: I91a9083be01348ec5a104303186e9e0e6cdbd8d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5401847


Reviewed-by: Nohemi Fernandez <fernandex@chromium.org>
Commit-Queue: Tomek Jurkiewicz <tju@google.com>
Cr-Commit-Position: refs/heads/main@{#1280235}