Add support for enabling DCHECKs in release mode

src/base/logging.h

Index: src/base/logging.h
diff --git a/src/base/logging.h b/src/base/logging.h
index 83c1bb60136c1ec9d626c0230db452f3dbb2bf96..9bfa74427942aebc26c78c92b3b8a53e6aaa6285 100644
--- a/src/base/logging.h
+++ b/src/base/logging.h
@@ -12,10 +12,16 @@
 
 extern "C" void V8_Fatal(const char* file, int line, const char* format, ...);
 
+#if defined(DEBUG) || defined(DCHECK_ALWAYS_ON)
+#define DCHECK_IS_ON 1
+#else
+#define DCHECK_IS_ON 0
+#endif
+
 
 // The FATAL, UNREACHABLE and UNIMPLEMENTED macros are useful during
 // development, but they should not be relied on in the final product.
-#ifdef DEBUG
+#if DCHECK_IS_ON
 #define FATAL(msg)                              \
   V8_Fatal(__FILE__, __LINE__, "%s", (msg))
 #define UNIMPLEMENTED()                         \
@@ -191,8 +197,8 @@ void DumpBacktrace();
 
 
 // The DCHECK macro is equivalent to CHECK except that it only
-// generates code in debug builds.
-#ifdef DEBUG
+// generates code in debug builds or if DCHECK_ALWAYS_ON is defined.
+#if DCHECK_IS_ON
 #define DCHECK_RESULT(expr)    CHECK(expr)
 #define DCHECK(condition)      CHECK(condition)
 #define DCHECK_EQ(v1, v2)      CHECK_EQ(v1, v2)