Perf sheriffs: this is going to increase GC time in v8.runtimestats
benchmarks. That time was not accounted before.

Change-Id: I656aed7ec7f4fd9f29dd4a2eff44eb25a60f21ad
Reviewed-on: https://chromium-review.googlesource.com/626636
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47520}