Additionally now we only scan over flat heap strings.

Change-Id: Ic449b19aecd7fc3f283a04a3df6a39772d471565
Reviewed-on: https://chromium-review.googlesource.com/1125854Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54224}