Skip logging spurious errors for ServiceWorker URLRequests that were spawned   through ServiceWorkerWriteToCacheJob. This CL does not fix the underlying issue, it is just suppressing a not so   useful error log.
Skip logging spurious errors for ServiceWorker URLRequests that were spawned
  through ServiceWorkerWriteToCacheJob.
This CL does not fix the underlying issue, it is just suppressing a not so
  useful error log.

BUG=568788
Test=Load an extension page with ServiceWorker, observe no
"Allowing load of ..." error messages anymore.

Review URL: https://codereview.chromium.org/1790993002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#381517}

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