[offthread] Enable off-thread finalization in --future

Enable the new LocalHeap-based off-thread finalization behind --future.

Bug: chromium:1011762
Change-Id: I4b33fc300a34530d75aa1a3e197e0d1326994efa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2403252
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69813}

src/flags/flag-definitions.h

@@ -392,6 +392,7 @@ DEFINE_BOOL(future, FUTURE_BOOL,
             "not-too-far future")
 
 DEFINE_IMPLICATION(future, write_protect_code_memory)
+DEFINE_IMPLICATION(future, finalize_streaming_on_background)
 
 DEFINE_BOOL(assert_types, false,
             "generate runtime type assertions to test the typer")