[d8] Use predictable platform if --predictable is passed

We currently only use the predictable platform if --verify-predictable
is passed, which is confusing and not following the comment on the
--predictable flag ("enable predictable mode").

This CL fixes that and makes --verify-predictable imply --predictable to
also allow to only pass --verify-predictable.

R=ahaas@chromium.org
CC=mlippautz@chromium.org

Bug: v8:11879
Change-Id: Ifb9683ddc4fab374ce519169533c90244175bb48
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3094010Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76305}

src/d8/d8.cc

@@ -5027,7 +5027,7 @@ int Shell::Main(int argc, char* argv[]) {
       options.thread_pool_size, v8::platform::IdleTaskSupport::kEnabled,
       in_process_stack_dumping, std::move(tracing));
   g_default_platform = g_platform.get();
-  if (i::FLAG_verify_predictable) {
+  if (i::FLAG_predictable) {
     g_platform = MakePredictablePlatform(std::move(g_platform));
   }
   if (options.stress_delay_tasks) {

src/flags/flag-definitions.h

@@ -2115,6 +2115,7 @@ DEFINE_NEG_IMPLICATION(single_threaded_gc, stress_concurrent_allocation)
 
 DEFINE_BOOL(verify_predictable, false,
             "this mode is used for checking that V8 behaves predictably")
+DEFINE_IMPLICATION(verify_predictable, predictable)
 DEFINE_INT(dump_allocations_digest_at_alloc, -1,
            "dump allocations digest each n-th allocation")