If the value can be converted, we can skip the write barrier.

Change-Id: I88ac7f3756ddfaf4b8e58dc36a9b26faf544f0d9
Reviewed-on: https://chromium-review.googlesource.com/957033
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51845}