The mutator utilizaton is computed for each mark-compact GC cycle as
mutator_time / total_time, where
- total_time is the time from the end of the previous GC to the end of
  the current GC
- mutator_time = total_time - incremental_steps_duration - gc_time.

Bug: chromium:824214
Change-Id: Ie1814f22f0816a3c9c579107f4950f6fc8c8a72d
Reviewed-on: https://chromium-review.googlesource.com/978215
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52221}