[test] Prepare making stress-marking mode explicit in gc fuzzer

This prepares to add the stress-marking flag on the infra side.

TBR=sergiyb@chromium.org
NOTRY=true

Bug: v8:6972
Change-Id: Ibee30beadb167d06fd7965dfd3cc05fb523158cb
Reviewed-on: https://chromium-review.googlesource.com/874350Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50697}

tools/testrunner/gc_fuzzer.py

@@ -71,6 +71,8 @@ class GCFuzzer(base_runner.BaseTestRunner):
     parser.add_option("--fuzzer-random-seed", default=0,
                       help="Default seed for initializing fuzzer random "
                       "generator")
+    parser.add_option("--stress-marking", default=False, action="store_true",
+                      help="Enable stress-marking flag")
     parser.add_option("--stress-compaction", default=False, action="store_true",
                       help="Enable stress_compaction_percentage flag")