[heap] Add basic infrastructure for incremental wrapper tracing.

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index ae095782c380c1c5cb0109dba436e9374260be82..e88ca8a076f86f561b66f65e951f2883e11fb2ee 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -2223,14 +2223,6 @@ void MarkCompactCollector::TracePossibleWrapper(JSObject* js_object) {
   }
 }
 
-void MarkCompactCollector::RegisterExternallyReferencedObject(Object** object) {
-  DCHECK(in_use());
-  HeapObject* heap_object = HeapObject::cast(*object);
-  DCHECK(heap_->Contains(heap_object));
-  MarkBit mark_bit = ObjectMarking::MarkBitFrom(heap_object);
-  MarkObject(heap_object, mark_bit);
-}
-
 class MarkCompactCollector::ObjectStatsVisitor
     : public MarkCompactCollector::HeapObjectVisitor {
  public: