Ship Notification action buttons by default

Intent to ship:
https://groups.google.com/a/chromium.org/d/topic/blink-dev/8iSlPpwokbY/discussion

BUG=513671,513672,548297

Review URL: https://codereview.chromium.org/1412223007

Cr-Commit-Position: refs/heads/master@{#358304}
7 files changed