[scheduler] Add metrics to capture amount of work per renderer runtime.

Add an experimental metric to measure how much work a single renderer
does during its lifetime.

Due to implementation constrains the real value should be calculated as
a difference of .Positive and .Negative histograms.

R=rkaplow@chromium.org,skyostil@chromium.org

Change-Id: I66ea1e56e9f9d27fef09a3f58d0e75bbfd68b5b0
Reviewed-on: https://chromium-review.googlesource.com/1133324
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Robert Kaplow (slow) <rkaplow@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575241}
8 files changed