Additionally now we only scan over flat heap strings.

Change-Id: Ia73c538a3c7923ec66089e16efa529ef3cea2d06
Reviewed-on: https://chromium-review.googlesource.com/1126938
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54258}