The existing script source tracking was racing with general external
script resource tracking, resulting in heap-order-dependent labels.

Bug: v8:8218
Change-Id: Ib09c28ca1e13768b328ecef7f8ee3ea5131ba12e
Reviewed-on: https://chromium-review.googlesource.com/1238927Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56191}