[heap] Start concurrent marking simultaneously with incremental marking.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index b50950e477ea93702649aec0d1b2f8caa4b503d8..64324a330b55db90e85c1a626bffaaa7ea3631bc 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1239,6 +1239,12 @@ class Heap {
 
   IncrementalMarking* incremental_marking() { return incremental_marking_; }
 
+  // ===========================================================================
+  // Concurrent marking API. ===================================================
+  // ===========================================================================
+
+  ConcurrentMarking* concurrent_marking() { return concurrent_marking_; }
+
   // The runtime uses this function to notify potentially unsafe object layout
   // changes that require special synchronization with the concurrent marker.
   // A layout change is unsafe if