Add support for enabling DCHECKs in release mode

src/heap/spaces.h

Index: src/heap/spaces.h
diff --git a/src/heap/spaces.h b/src/heap/spaces.h
index dcd336437e9cdc161ff5b157a3f1de512a4e051e..e44c4a42fc0c266eea08aed61b387e2df212243b 100644
--- a/src/heap/spaces.h
+++ b/src/heap/spaces.h
@@ -108,7 +108,7 @@ class MarkBit {
   inline CellType* cell() { return cell_; }
   inline CellType mask() { return mask_; }
 
-#ifdef DEBUG
+#if DCHECK_IS_ON
   bool operator==(const MarkBit& other) {
     return cell_ == other.cell_ && mask_ == other.mask_;
   }
@@ -1491,7 +1491,7 @@ class FreeListCategory {
 
   bool IsEmpty() { return top() == 0; }
 
-#ifdef DEBUG
+#if DCHECK_IS_ON
   intptr_t SumFreeList();
   int FreeListLength();
 #endif
@@ -1579,7 +1579,7 @@ class FreeList {
            large_list_.IsEmpty() && huge_list_.IsEmpty();
   }
 
-#ifdef DEBUG
+#if DCHECK_IS_ON
   void Zap();
   intptr_t SumFreeLists();
   bool IsVeryLong();