[Background Sync] Update UMA recording logic.

Add the newly added Background Task to BackgroundTaskId enum
and update the UMA recording logic to derive UMA value from TaskID.

Bug: 924490,931795,931705
Change-Id: Ibc2c2a1d01906752fc43e9cef6ebbf0a85176d7e
Reviewed-on: https://chromium-review.googlesource.com/c/1472717
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632569}
3 files changed