Revert of service worker: use 200 OK for update requests even in the no update...

Revert of service worker: use 200 OK for update requests even in the no update case (patchset #8 id:140001 of https://codereview.chromium.org/1675613002/ )

Reason for revert:
https://build.chromium.org/p/chromium.memory/builders/Linux%20ASan%20LSan%20Tests%20%281%29?numbuilds=200 got much more flaky after this landed. Speculatively revert and see if it helps. See also http://crbug.com/586897

Original issue's description:
> service worker: use 200 OK for update requests even in the no update case
>
> The main motivation is to stop network failure error spew in DevTools console.
> This patch reinstates the "pause on download" functionality we once had.
>
> BUG=541797
>
> Committed: https://crrev.com/42bc9253c79fa01fc6a6af5b3908dcc063a12023
> Cr-Commit-Position: refs/heads/master@{#376032}

TBR=tkent@chromium.org,michaeln@chromium.org,nhiroki@chromium.org,tsepez@chromium.org,falken@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=541797

Review URL: https://codereview.chromium.org/1710903002

Cr-Commit-Position: refs/heads/master@{#376161}