Add metric for ARC Total engagement time, which is the sum of foreground and background.

Bug: 936355
Change-Id: I5d9e685ca063be7af28cf999adf249766209f911
Reviewed-on: https://chromium-review.googlesource.com/c/1491232
Commit-Queue: Maajid <maajid@chromium.org>
Reviewed-by: Shao-Chuan Lee <shaochuan@chromium.org>
Reviewed-by: Christopher Thompson <cthomp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636710}
3 files changed