Add support for enabling DCHECKs in release mode

src/handles.h

Index: src/handles.h
diff --git a/src/handles.h b/src/handles.h
index eb57f0e26089383c9d7fe9f57a8cba5f41a111e2..6a485af026d923607bc2172ceaebae78b3caeb9b 100644
--- a/src/handles.h
+++ b/src/handles.h
@@ -128,11 +128,11 @@ class Handle {
   // implementation in api.h.
   inline Handle<T> EscapeFrom(v8::EscapableHandleScope* scope);
 
-#ifdef DEBUG
+#ifdef ENABLE_SLOW_DCHECKS
   enum DereferenceCheckMode { INCLUDE_DEFERRED_CHECK, NO_DEFERRED_CHECK };
 
   bool IsDereferenceAllowed(DereferenceCheckMode mode) const;
-#endif  // DEBUG
+#endif  // ENABLE_SLOW_DCHECKS
 
  private:
   T** location_;
@@ -256,7 +256,7 @@ class DeferredHandleScope {
   Object** prev_next_;
   HandleScopeImplementer* impl_;
 
-#ifdef DEBUG
+#if DCHECK_IS_ON
   bool handles_detached_;
   int prev_level_;
 #endif