Measure remembered set processing during Scavenge.

Change-Id: I8f7f31269bf1c09ff1f3cb3455fc96bce5c4f35d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1895562
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64755}