[Extensions][TaskScheduler] Update ActivityLog for scheduling migration

Migrate the ActivityLog and related classes away from posting tasks to
and checking that they are currently on the content::BrowserThread::DB
thread, and replace this with a singleton SequencedTaskRunner to be used
in the activity log.

BUG=689520

Review-Url: https://codereview.chromium.org/2980503002
Cr-Commit-Position: refs/heads/master@{#491137}
14 files changed