Currently transition array targets have conditional weakness depending
on the type of the target. Map targets are weak and all other targets
are strong. This patch wraps maps in transitions arrays in weak cells,
which allows us to treat all elements of transition arrays strongly.

Conditional weakness is unsafe for concurrent marking because the
condition can change during marking.

Bug: chromium:694255
Change-Id: I64e5d0699698fc7c1758f3fbc52da43014c247af
Reviewed-on: https://chromium-review.googlesource.com/641271
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48034}