Turn on DCHECKs and other debugging code if dcheck_always_on is 1

src/checks.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_CHECKS_H_
 #define V8_CHECKS_H_
 
 #include "src/base/logging.h"
 
-#ifdef DEBUG
-#ifndef OPTIMIZED_DEBUG
-#define ENABLE_SLOW_DCHECKS    1
-#endif
-#endif
-
 namespace v8 {
 
 class Value;
 template <class T> class Handle;
 
 namespace internal {
 
 intptr_t HeapObjectTagMask();
 
 #ifdef ENABLE_SLOW_DCHECKS
@@ skipping 27 matching lines @@
                        v8::Handle<v8::Value> expected,
                        const char* value_source,
                        v8::Handle<v8::Value> value);
 
 #define DCHECK_TAG_ALIGNED(address) \
   DCHECK((reinterpret_cast<intptr_t>(address) & HeapObjectTagMask()) == 0)
 
 #define DCHECK_SIZE_TAG_ALIGNED(size) DCHECK((size & HeapObjectTagMask()) == 0)
 
 #endif  // V8_CHECKS_H_