Now the concurrent marker skips the weak fields of AllocationSites.

BUG=chromium:694255

Change-Id: If920b6a14fa12c3a90ab0d283e22988ccbd48c0e
Reviewed-on: https://chromium-review.googlesource.com/574907
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46720}