Don't share Push API message payloads unless explicitly enabled.

There are open security issues about exposing payloads for the Push API,
so we'd like to guard those behind a command line flag for now.

This is part of a three-sided patch:
  [1] https://codereview.chromium.org/872013002
  [2] This patch.
  [3] https://codereview.chromium.org/869303002

BUG=449184

Review URL: https://codereview.chromium.org/874613002

Cr-Commit-Position: refs/heads/master@{#313265}
5 files changed