When FLAG_shared_string_table is true, postMessaging strings will share
instead of copy.

Note that not all operations on shared strings are supported, and shared
strings may be slower than non-shared strings for some operations.

Bug: v8:12007
Change-Id: I3462128e15410d2568868143571571b3025722c1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3277250Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78614}