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}
(cherry picked from commit d039981e6de2c11e1dfdc1c0f6ba61804cc92c49)

Review-Url: https://codereview.chromium.org/2704573002 .
Cr-Commit-Position: refs/branch-heads/2987@{#555}
Cr-Branched-From: ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}
4 files changed