ExternalStrings in the shared heap currently conflicts with the sandbox
project. We would need concurrent concurrent allocation in the external
pointer table but also require different accessors for them.

Since the shared string table doesn't really need ExternalStrings in
the shared heap for now, simply keep ExternalStrings in the client
heaps.

Bug: v8:11708, v8:12617
Change-Id: I272e40eaec4b7f368ce44f42f7f69bf27d53f9c7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3451717Reviewed-by: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79039}