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

Bug: chromium:1011762
Change-Id: I3dc2975f3a93fa4c780e54e6e5bd8e689e008f36
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2190751
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67684}

src/flags/flag-definitions.h

@@ -374,6 +374,7 @@ 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")