This adds the following histograms:
- V8.GCBackgroundMarking
- V8.GCBackgroundScavenger
- V8.GCBackgroundSweeping

Bug: chromium:792552
Change-Id: Iae6fa3258f4fe0d4ed5e415c541a6d29101893a9
Reviewed-on: https://chromium-review.googlesource.com/819530Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50005}