[heap] Start concurrent marking simultaneously with incremental marking.

src/heap/mark-compact.h

Index: src/heap/mark-compact.h
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index 254748174fdfd3f1c1210b9bd8345f1b01c3c71a..8041bfe3eaebd7be66eba7cb60117b1d25963b46 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -177,6 +177,15 @@ class MarkingDeque {
     }
   }
 
+  template <typename Callback>
+  void Iterate(Callback callback) {
+    int i = bottom_;
+    while (i != top_) {
+      callback(array_[i]);
+      i = (i + 1) & mask_;
+    }
+  }
+
   HeapObject** array() { return array_; }
   int bottom() { return bottom_; }
   int top() { return top_; }