Allow Blink to respond asynchronously to SW Push events (side 1/3)

This will be necessary once Push events support event.waitUntil to keep
the Service Worker alive until the Promise is resolved
(https://github.com/w3c/push-api/issues/85).

Required by Blink https://codereview.chromium.org/709103002

BUG=430888, 401438

Review URL: https://codereview.chromium.org/708913003

Cr-Commit-Position: refs/heads/master@{#303365}
2 files changed