marking visitors.

This makes incremental and concurrent visitors of share function infos
side-effect free.

BUG=chromium:694255

Change-Id: I85ee7bac17f17bdbc101ef64ecfb46020b5b3458
Reviewed-on: https://chromium-review.googlesource.com/574851
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46796}