Implement independent write barrier for contents of objects.

Change-Id: I739745f524245fa61b2b1cd01263cc8fe3a3d373
Reviewed-on: https://chromium-review.googlesource.com/1152909
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54979}