[heap] Add --stress-incremental-marking flag.

test/cctest/heap/test-incremental-marking.cc

Index: test/cctest/heap/test-incremental-marking.cc
diff --git a/test/cctest/heap/test-incremental-marking.cc b/test/cctest/heap/test-incremental-marking.cc
index ce1fb349517f4868e24703c982916bef9a7d8f38..f5cefe555b9666e9de3f9965c147a1c0b7b2584c 100644
--- a/test/cctest/heap/test-incremental-marking.cc
+++ b/test/cctest/heap/test-incremental-marking.cc
@@ -101,6 +101,7 @@ class MockPlatform : public v8::Platform {
 
 TEST(IncrementalMarkingUsingTasks) {
   if (!i::FLAG_incremental_marking) return;
+  FLAG_stress_incremental_marking = false;
   CcTest::InitializeVM();
   v8::Platform* old_platform = i::V8::GetCurrentPlatform();
   MockPlatform platform(old_platform);