Currently they are using a generic IterateBody(ObjectVisit*), which has
an overhead of virtual table lookup for each visited pointer.

Change-Id: I97268bf7fe63f8c99834d5fc31b4ce18a0fa5655
Reviewed-on: https://chromium-review.googlesource.com/979437
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52220}