[Background Sync] Add Periodic Sync wakeup task

This adds a BackgroundTaskScheduler task to wake up Chrome
to process Periodic Background Sync registrations.

The logic to actually schedule this task will be added in
follow-up CLs.

Bug: 925297

Change-Id: Id8a3b8804b2b6f66bbb206832318fada8bffc3f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1663495
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Auto-Submit: Mugdha Lakhani <nator@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Richard Knoll <knollr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672028}
21 files changed