[Background Sync] Add UMA for wake up task

to note whether or not any relevant sync events were
fired as a result of waking up Chrome.

Bug: 995795
Change-Id: I2e0da8c3df47c3e558264314a1c3aec22f451cdf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1762237
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688971}