Add support for enabling DCHECKs in release mode

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index cc0f70bf318c5f57fca3e49c5d5c9d7daaf9a3ff..3ed2f0709226f5a985dfedebd80576abc9d97622 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -7567,7 +7567,7 @@ char* HandleScopeImplementer::RestoreThread(char* storage) {
 
 
 void HandleScopeImplementer::IterateThis(ObjectVisitor* v) {
-#ifdef DEBUG
+#if DCHECK_IS_ON
   bool found_block_before_deferred = false;
 #endif
   // Iterate over all handles in the blocks except for the last.
@@ -7578,7 +7578,7 @@ void HandleScopeImplementer::IterateThis(ObjectVisitor* v) {
         (last_handle_before_deferred_block_ >= block)) {
       v->VisitPointers(block, last_handle_before_deferred_block_);
       DCHECK(!found_block_before_deferred);
-#ifdef DEBUG
+#if DCHECK_IS_ON
       found_block_before_deferred = true;
 #endif
     } else {