Disable FileReaderSync in service workers.

Usage so far in service workers is zero, so go ahead with disabling
FileReaderSync in service workers as planned.

Intent to deprecate and remove:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/cjWtqRD6iw8/hcxtAeE2CAAJ

BUG=688586

Review-Url: https://codereview.chromium.org/2753943002
Cr-Commit-Position: refs/heads/master@{#458487}
8 files changed