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

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index b9f633a43990dc6fef572677f5cdca8a7bb718c8..d0fb9a4ff1ff793ce75dabcad6daec7f092245af 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -5544,9 +5544,15 @@ bool Heap::ShouldExpandOldGenerationOnSlowAllocation() {
 Heap::IncrementalMarkingLimit Heap::IncrementalMarkingLimitReached() {
   // Code using an AlwaysAllocateScope assumes that the GC state does not
   // change; that implies that no marking steps must be performed.
-  if (!incremental_marking()->CanBeActivated() || always_allocate() ||
-      PromotedSpaceSizeOfObjects() <=
-          IncrementalMarking::kActivationThreshold) {
+  if (!incremental_marking()->CanBeActivated() || always_allocate()) {
+    // Incremental marking is disabled or it is too early to start.
+    return IncrementalMarkingLimit::kNoLimit;
+  }
+  if (FLAG_stress_incremental_marking) {
+    return IncrementalMarkingLimit::kHardLimit;
+  }
+  if (PromotedSpaceSizeOfObjects() <=
+      IncrementalMarking::kActivationThreshold) {
     // Incremental marking is disabled or it is too early to start.
     return IncrementalMarkingLimit::kNoLimit;
   }