Tip of tree puts both internalized and in-place-internalizable strings
into the shared heap object cache. But only internalized strings need
to go in there, since we can't have duplicates of those. It's fine to
allocate in-place-internalizable strings in the shared heap each time
a new Isolate is initialized, it'll be deduplicated if it's
internalized eventually.

Bug: chromium:1258918, v8:12007
Change-Id: I0e46b73a5ac3be83d0eaa31915a3a24f47a8c2bd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3219690
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77388}