[Background Sync] Allow native processing to finish

sending any pending Background Sync events, before exiting the
background task that woke up the browser.

Bug: 924490
Change-Id: Id63b2d9edeb6e9255dff616ea06f02c9f77cc0d3
Reviewed-on: https://chromium-review.googlesource.com/c/1477707
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636034}
25 files changed