[heap] Implement simple concurrent marking deque.

src/heap/incremental-marking.cc

Index: src/heap/incremental-marking.cc
diff --git a/src/heap/incremental-marking.cc b/src/heap/incremental-marking.cc
index 5767ad5596e1db27680b5dc8b4670c83a82a46e8..856819d37df7a40b5dea7dc79b65bbf18e2ecab7 100644
--- a/src/heap/incremental-marking.cc
+++ b/src/heap/incremental-marking.cc
@@ -551,9 +551,6 @@ void IncrementalMarking::StartMarking() {
 
   if (FLAG_concurrent_marking) {
     ConcurrentMarking* concurrent_marking = heap_->concurrent_marking();
-    marking_deque()->Iterate([concurrent_marking](HeapObject* obj) {
-      concurrent_marking->AddRoot(obj);
-    });
     concurrent_marking->StartTask();
   }