And also ensure that it doesn't allocate when that's avoidable.

Bug: v8:6791
Change-Id: Ied2d2c9c5718aed9839f17739d743353cbadee88
Reviewed-on: https://chromium-review.googlesource.com/786170Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49683}