Deprecate FileReaderSync in service workers.

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/2680363003
Cr-Commit-Position: refs/heads/master@{#450593}
4 files changed