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

Bug: chromium:792552
Change-Id: I15f3df49773b630620d6c67121c46ad7fff3e179
Reviewed-on: https://chromium-review.googlesource.com/819610Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523754}