Allow BackoffEntry to be serialized and deserialized.

This will be used by push messaging for https://crbug.com/465399,
where we need to continue retrying unregistration even if the browser
gets killed and restarted.

BUG=465399

Review URL: https://codereview.chromium.org/1023473003

Cr-Commit-Position: refs/heads/master@{#328755}
6 files changed