The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d065863ffa29ca63d569b6555a0e9697c90def8d

commit d065863ffa29ca63d569b6555a0e9697c90def8d
Author: jkarlin <jkarlin@chromium.org>
Date: Wed Mar 16 21:05:23 2016

[BackgroundSync] Remove RegistrationKey

Now that periodic syncs are out of the BackgroundSyncManager we no
longer need the RegistrationKey and can index registrations by tag.

BUG= 592599 

Review URL: https://codereview.chromium.org/1774603002

Cr-Commit-Position: refs/heads/master@{#381542}

[modify] https://crrev.com/d065863ffa29ca63d569b6555a0e9697c90def8d/content/browser/background_sync/background_sync_manager.cc
[modify] https://crrev.com/d065863ffa29ca63d569b6555a0e9697c90def8d/content/browser/background_sync/background_sync_manager.h