With String contents being accessible off-main-thread or from multiple
main threads, add a SLOW_DCHECK that the hash of the string contents
inside a String::FlatContent doesn't change during its lifetime.

Bug: v8:12007
Change-Id: Iaf6bb785e44c97c13ac2fe9c5c20099bf1e0d2fd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3451355Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79080}