Since the string-table's data pointer is written inside a mutex, but
read outside of it, we need to acq/rel access to it.

There's no support in C++ for an std::atomic<std::unique_ptr>, so this
patch changes the std::unique_ptr<Data> into a std::atomic<Data*>, and
handles the deletion manually. StringTable::Data still uses

std::unique_ptr as the general pointer-passing contract, we just
carefully set and release the unique_ptrs when accessing and setting
the StringTable's atomic Data pointer.

Change-Id: I711a56825e2f5f9b2db63d1874e09c2627af54b8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2410057
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69932}