Use absolute value for intermediate AverageLag values

Area values generally should not be negative. With the existing code,
changes of direction during a report period will cancel out previously
accumulated lag.

Bug: 928843

Change-Id: I779d16b08e47c38fe0c33e807b5d8241fd5110cb
Reviewed-on: https://chromium-review.googlesource.com/c/1450751
Reviewed-by: Ella Ge <eirage@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Commit-Queue: Daniel Libby <dlibby@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#629702}
2 files changed