[heap] Add basic infrastructure for incremental wrapper tracing.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index 9d6204df2a53974b81b37bb965a1ae2f288aa7ea..8178a1ab4854217aa3986576f3d4b47c4c8eb2bb 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -5454,7 +5454,15 @@ void Heap::TracePossibleWrapper(JSObject* js_object) {
 }
 
 void Heap::RegisterExternallyReferencedObject(Object** object) {
-  mark_compact_collector()->RegisterExternallyReferencedObject(object);
+  HeapObject* heap_object = HeapObject::cast(*object);
+  DCHECK(Contains(heap_object));
+  if (FLAG_incremental_marking_wrappers && incremental_marking()->IsMarking()) {
+    IncrementalMarking::MarkGrey(this, heap_object);
+  } else {
+    DCHECK(mark_compact_collector()->in_use());
+    MarkBit mark_bit = ObjectMarking::MarkBitFrom(heap_object);
+    mark_compact_collector()->MarkObject(heap_object, mark_bit);
+  }
 }
 
 void Heap::TearDown() {