The race happens during evacuation when multiple threads access the
main space capacity to check CanExpandOldGeneration.

Bug: chromium:694255
Change-Id: I63dbb71cc3a894f85ee11411a5dc01d53daefa11
Reviewed-on: https://chromium-review.googlesource.com/618876
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47414}