[test] Set fuzzer seed to random seed by default

Bug: v8:6917
Change-Id: Iab4e9d7a8f8a4313a6722610fb6a2cfcff703d81
Reviewed-on: https://chromium-review.googlesource.com/866710Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michał Majewski <majeski@google.com>
Cr-Commit-Position: refs/heads/master@{#50573}

src/flag-definitions.h

@@ -984,7 +984,7 @@ DEFINE_INT(random_seed, 0,
            "(0, the default, means to use system random).")
 DEFINE_INT(fuzzer_random_seed, 0,
            "Default seed for initializing fuzzer random generator "
-           "(0, the default, means to use system random).")
+           "(0, the default, means to use v8's random number generator seed).")
 DEFINE_BOOL(trace_rail, false, "trace RAIL mode")
 DEFINE_BOOL(print_all_exceptions, false,
             "print exception object and stack trace on each thrown exception")

src/isolate.cc

@@ -3455,7 +3455,16 @@ base::RandomNumberGenerator* Isolate::random_number_generator() {
 }
 
 base::RandomNumberGenerator* Isolate::fuzzer_rng() {
-  return ensure_rng_exists(&fuzzer_rng_, FLAG_fuzzer_random_seed);
+  if (fuzzer_rng_ == nullptr) {
+    int64_t seed = FLAG_fuzzer_random_seed;
+    if (seed == 0) {
+      seed = random_number_generator()->initial_seed();
+    }
+
+    fuzzer_rng_ = new base::RandomNumberGenerator(seed);
+  }
+
+  return fuzzer_rng_;
 }
 
 int Isolate::GenerateIdentityHash(uint32_t mask) {