- Make it explicit that the field marks are supposed to be all-false after each
  object.

- Remove unused param from MarkVisitedField.

BUG=v8:7534

Change-Id: Ibc226290bb09daca60b92819944e0049bb33e594
Reviewed-on: https://chromium-review.googlesource.com/951725Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51799}