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This adds SERVICE_WORKER_ERROR_DISK_CACHE to indicate
failure to read from the disk cache. It also re-enables
logging of StartWorker for redundant workers. This was
previously disabled because we got a lot of "failed" spam
for updates that get aborted when there is no script change,
so I've changed StartWorker to fail with SW_ERROR_EXISTS in
that case.

This patch is a step in the direction of evicting a SW
that can't be read from disk cache.

BUG=448003

Review URL: https://codereview.chromium.org/1054033004

Cr-Commit-Position: refs/heads/master@{#326923}