Use correct type for the map-counters flag used in d8.

src/d8.cc

 // Copyright 2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 387 matching lines @@
 
 void Shell::AddHistogramSample(void* histogram, int sample) {
   Counter* counter = reinterpret_cast<Counter*>(histogram);
   counter->AddSample(sample);
 }
 
 
 void Shell::Initialize() {
   Shell::counter_map_ = new CounterMap();
   // Set up counters
-  if (i::FLAG_map_counters != NULL)
+  if (i::StrLength(i::FLAG_map_counters) != 0)
     MapCounters(i::FLAG_map_counters);
   if (i::FLAG_dump_counters) {
     V8::SetCounterFunction(LookupCounter);
     V8::SetCreateHistogramFunction(CreateHistogram);
     V8::SetAddHistogramSampleFunction(AddHistogramSample);
   }
 
   // Initialize the global objects
   HandleScope scope;
   Handle<ObjectTemplate> global_template = ObjectTemplate::New();
@@ skipping 358 matching lines @@
   return 0;
 }
 
 
 }  // namespace v8
 
 
 int main(int argc, char* argv[]) {
   return v8::Shell::Main(argc, argv);
 }