Only Internalized and Thin strings should be accessed from the
background. My CL "[compiler] Remove map check in StringRef::length"
removed a check, and, while doing so, made it possible to read Cons
strings in the background, which is incorrect because such strings can
be mutated by the main threads.

Fixed: chromium:1357438
Change-Id: I3a5445f91d10e5ceab62ac208a04be5ed71798ff
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3862007
Commit-Queue: Darius Mercadier <dmercadier@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82813}