This fixes a TSAN data race when writing the forwarding pointer in
MigrateObject and reading the object as a LayoutDescriptor when trying
to figure out the layout of another object in parallel.

BUG=chromium:701732

Change-Id: I1e291fa1afb42771244e1346680164de71c3a838
Reviewed-on: https://chromium-review.googlesource.com/455817Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43826}