FLAG_max_new_space_size is in MB.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 41b5a45a70ebf78e86d271b1c68df2ddd29dd1d8..1fc270c7255ceddbdae4557714de08a7562a15c1 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -474,15 +474,15 @@ bool SetResourceConstraints(Isolate* v8_isolate,
                             ResourceConstraints* constraints) {
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(v8_isolate);
   int new_space_size = constraints->max_new_space_size();
-  int old_gen_size = constraints->max_old_space_size();
+  int old_space_size = constraints->max_old_space_size();
   int max_executable_size = constraints->max_executable_size();
   int code_range_size = constraints->code_range_size();
-  if (new_space_size != 0 || old_gen_size != 0 || max_executable_size != 0 ||
+  if (new_space_size != 0 || old_space_size != 0 || max_executable_size != 0 ||
       code_range_size != 0) {
     // After initialization it's too late to change Heap constraints.
     ASSERT(!isolate->IsInitialized());
     bool result = isolate->heap()->ConfigureHeap(new_space_size / 2,
-                                                 old_gen_size,
+                                                 old_space_size,
                                                  max_executable_size,
                                                  code_range_size);
     if (!result) return false;