Push API: Grace - allow one in ten pushes to show no notification.

Push API: Grace - allow one in ten pushes to show no notification.

For developers who have opted in to showing a user-visible UI change on
every push (in exchange for showing the user a less scary permission
prompt), we currently[1] enforce that each push shows a notification.

This patch allows one in ten pushes to show no notification, so
occasional bugs in developer push handlers don't needlessly spam the
user.

Specifically, it keeps track of whether the last (up to) 10 push
messages showed a notification (ignoring pushes didn't show a
notification but were exempt, e.g. due to https://crrev.com/866443003)
and only shows a forced notification if one of those (up to) 10 also
failed to show a notification.

[1]: (since https://crrev.com/842233003)

BUG=437277

Review URL: https://codereview.chromium.org/883743002

Cr-Commit-Position: refs/heads/master@{#314851}