Ensure black and gray objects are kept around by scavenger

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index a358feafe592c488ee1e0681da2e08790bde2945..8d605317090a0d3e6611640e754dfddc73e8618c 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -1706,6 +1706,16 @@ void Heap::Scavenge() {
     new_space_front = DoScavenge(&scavenge_visitor, new_space_front);
   }
 
+  {
+    // Scavenge objects marked black or grey by MarkCompact collector.
+    TRACE_GC(tracer(),
+             GCTracer::Scope::SCAVENGER_OBJECTS_MARKED_BY_MARK_COMPACT);
+    if (incremental_marking()->IsMarking()) {
+      PromoteMarkedObjects();
+      new_space_front = DoScavenge(&scavenge_visitor, new_space_front);
+    }
+  }
+
   if (FLAG_scavenge_reclaim_unmodified_objects) {
     isolate()->global_handles()->MarkNewSpaceWeakUnmodifiedObjectsPending(
         &IsUnscavengedHeapObject);
@@ -3069,6 +3079,7 @@ void Heap::CreateFillerObjectAt(Address addr, int size,
   if (mode == ClearRecordedSlots::kYes) {
     ClearRecordedSlotRange(addr, addr + size);
   }
+
   // At this point, we may be deserializing the heap from a snapshot, and
   // none of the maps have been created yet and are NULL.
   DCHECK((filler->map() == NULL && !deserialization_complete_) ||
@@ -4854,6 +4865,21 @@ void Heap::IterateStrongRoots(ObjectVisitor* v, VisitMode mode) {
   // checking of the sync flag in the snapshot would fail.
 }
 
+void Heap::PromoteMarkedObjects() {
+  NewSpacePageIterator it(new_space_.from_space());
+  while (it.has_next()) {
+    Page* page = it.next();
+    LiveObjectIterator<kAllLiveObjects> it(page);
+    Object* object = nullptr;
+    while ((object = it.Next()) != nullptr) {
+      if (object->IsHeapObject()) {
+        HeapObject* heap_object = HeapObject::cast(object);
+        DCHECK(IsUnscavengedHeapObject(this, &object));
+        Scavenger::ScavengeObject(&heap_object, heap_object, FORCE_PROMOTION);
+      }
+    }
+  }
+}
 
 // TODO(1236194): Since the heap size is configurable on the command line
 // and through the API, we should gracefully handle the case that the heap