Special case script logging to also log the source of the script, even
if that source is off-heap in an external string.

Bug: v8:7266
Change-Id: I0d35f94f7b27d0d793d1a1a3fb8d3280960b253d
Reviewed-on: https://chromium-review.googlesource.com/899344
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51076}