[BackgroundSync] Enable BackgroundSync by default

Intent to ship: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/t9apD7cQb6I

BUG=449443

Review URL: https://codereview.chromium.org/1514383002

Cr-Commit-Position: refs/heads/master@{#364709}
5 files changed