[Background Sync] UMA for Periodic Background Sync

Add UMA for Periodic Background Sync + a unit test
to verify expected metric collection.

Bug: 925297
Change-Id: Iaaae0ea2e224e89fc33869c605e4d0fec74bda08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1706697
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@{#679284}