Add support for enabling DCHECKs in release mode

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 e5ab308bef178834825423b2f9181f29a7a99b77..249fb29caf5c5cd4264ecd545ed81496f7aa0bca 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -80,7 +80,7 @@ class Marking {
 
   void TransferMark(Address old_start, Address new_start);
 
-#ifdef DEBUG
+#if DCHECK_IS_ON
   enum ObjectColor {
     BLACK_OBJECT,
     WHITE_OBJECT,
@@ -523,7 +523,7 @@ class MarkCompactCollector {
 
   void AbortCompaction();
 
-#ifdef DEBUG
+#if DCHECK_IS_ON
   // Checks whether performing mark-compact collection.
   bool in_use() { return state_ > PREPARE_GC; }
   bool are_map_pointers_encoded() { return state_ == UPDATE_POINTERS; }
@@ -676,7 +676,7 @@ class MarkCompactCollector {
 
   void StartSweeperThreads();
 
-#ifdef DEBUG
+#if DCHECK_IS_ON
   enum CollectorState {
     IDLE,
     PREPARE_GC,
@@ -875,14 +875,6 @@ class MarkCompactCollector {
   // Updates store buffer and slot buffer for a pointer in a migrating object.
   void RecordMigratedSlot(Object* value, Address slot);
 
-#ifdef DEBUG
-  friend class MarkObjectVisitor;
-  static void VisitObject(HeapObject* obj);
-
-  friend class UnmarkObjectVisitor;
-  static void UnmarkObject(HeapObject* obj);
-#endif
-
   Heap* heap_;
   base::VirtualMemory* marking_deque_memory_;
   bool marking_deque_memory_committed_;