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This does the remaining work to add BackgroundSync support.
An end to end browsertest has been added to verify the registration +
sync event firing.

Also, manually tested using the site:
https://webplatformapis.com/one_off_sync/one_off_sync.html

Note that WebLayer doesn't yet wake the browser up to fire sync
events, as the only user of WebLayer doesn't need this functionality.

Bug: 1087486
Change-Id: Iceb8c3a4cbcf4f0f8611905b07c950ac069eb6da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2476282Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820155}