[foozzie] Add flag experiments for gc fuzzing

This will add three gc fuzzing flags with 5% likelihood each to the second
correctness fuzzing config. The random checks are determined by the
top-level random-seed passed to the script.

This change depends on setting the fuzzer random seed as default to the
standard random seed, since the former isn't explicitly passed by
clusterfuzz.

NOTRY=true
TBR=hpayer@chromium.org

Bug: v8:7012
Change-Id: I794dc48bb953b6a95bbc4fc4305ad561bc13b6ee
Reviewed-on: https://chromium-review.googlesource.com/865912Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50577}

tools/foozzie/testdata/failure_output.txt

@@ -11,7 +11,7 @@
 # Flags of x64,ignition:
 --abort_on_stack_or_string_length_overflow --expose-gc --allow-natives-syntax --invoke-weak-callbacks --omit-quit --es-staging --random-seed 12345 --turbo-filter=~ --noopt --suppress-asm-messages
 # Flags of x64,ignition_turbo:
---abort_on_stack_or_string_length_overflow --expose-gc --allow-natives-syntax --invoke-weak-callbacks --omit-quit --es-staging --random-seed 12345 --suppress-asm-messages
+--abort_on_stack_or_string_length_overflow --expose-gc --allow-natives-syntax --invoke-weak-callbacks --omit-quit --es-staging --random-seed 12345 --suppress-asm-messages --stress-scavenge=100
 #
 # Difference:
 - unknown

tools/foozzie/v8_foozzie.py

@@ -12,6 +12,7 @@ import hashlib
 import itertools
 import json
 import os
+import random
 import re
 import sys
 import traceback
@@ -75,6 +76,14 @@ CONFIGS = dict(
   ],
 )
 
+# Additional flag experiments. List of tuples like
+# (<likelihood to use flags in [0,1)>, <flag>).
+ADDITIONAL_FLAGS = [
+  (0.05, '--stress-marking=100'),
+  (0.05, '--stress-scavenge=100'),
+  (0.05, '--stress-compaction-random'),
+]
+
 # Timeout in seconds for one d8 run.
 TIMEOUT = 3
 
@@ -247,6 +256,7 @@ def fail_bailout(output, ignore_by_output_fun):
 
 def main():
   options = parse_args()
+  rng = random.Random(options.random_seed)
 
   # Suppressions are architecture and configuration specific.
   suppress = v8_suppressions.get_suppression(
@@ -267,6 +277,11 @@ def main():
   first_config_flags = common_flags + CONFIGS[options.first_config]
   second_config_flags = common_flags + CONFIGS[options.second_config]
 
+  # Add additional flags to second config based on experiment percentages.
+  for p, flag in ADDITIONAL_FLAGS:
+    if rng.random() < p:
+      second_config_flags.append(flag)
+
   def run_d8(d8, config_flags):
     preamble = PREAMBLE[:]
     if options.first_arch != options.second_arch: