SeqStrings have their padding bytes serialized as 0s for deterministic
snapshot contents. Currently this is done by mutating the SeqStrings and
memsetting their padding bytes to 0 when serializing. This mutation is
not threadsafe in the presence of shared strings. This CL removes the
mutation by serializing the data and padding payloads separately for
SeqStrings.

Bug: v8:12939
Change-Id: I58c3ada767ce41e0a874a2d6e6392a86142fa1e1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3715715Reviewed-by: Patrick Thier <pthier@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81334}