Add responsiveness calculator.

This is the first step in implementing JankyIntervalsPerThirtySeconds. Future
CLs will forward task and event execution latency to this class.

Change-Id: Ib695f7761505a0ca9ac7911fef444d479f40574f
Bug: 859155
Reviewed-on: https://chromium-review.googlesource.com/1120754
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576965}
7 files changed