Add support for enabling DCHECKs in release mode

src/json-parser.h

Index: src/json-parser.h
diff --git a/src/json-parser.h b/src/json-parser.h
index 5ebbcdd861b351bc072d08ba209037bc1ed36678..40c381a87ea1c20be013526068e86b244e59e18c 100644
--- a/src/json-parser.h
+++ b/src/json-parser.h
@@ -348,7 +348,7 @@ Handle<Object> JsonParser<seq_one_byte>::ParseJsonObject() {
       }
 
       position_ = start_position;
-#ifdef DEBUG
+#if DCHECK_IS_ON
       c0_ = '"';
 #endif
 
@@ -769,7 +769,7 @@ Handle<String> JsonParser<seq_one_byte>::ScanJsonString() {
       if (element != isolate()->heap()->the_hole_value() &&
           String::cast(element)->IsOneByteEqualTo(string_vector)) {
         result = Handle<String>(String::cast(element), isolate());
-#ifdef DEBUG
+#if DCHECK_IS_ON
         uint32_t hash_field =
             (hash << String::kHashShift) | String::kIsNotArrayIndexMask;
         DCHECK_EQ(static_cast<int>(result->Hash()),