[BackgroundSyncManager] Tags with different periodicities can overlap
The background sync spec (see https://github.com/slightlyoff/BackgroundSync/pull/74) allows for
registrations to coexist with the same tag so long as they have different periodicities (one-shot vs periodic).
This CL changes Registration::fire_once to Registration::periodicity and indexes registrations by RegistrationKey instead of by tag. RegistrationKey incorporates both the tag and the periodicity.
BUG=474573
Review URL: https://codereview.chromium.org/
1065313002
Cr-Commit-Position: refs/heads/master@{#324691}