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

src/flag-definitions.h

 // Copyright 2010 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 337 matching lines @@
 // Dev shell flags
 //
 
 DEFINE_bool(help, false, "Print usage message, including flags, on console")
 DEFINE_bool(dump_counters, false, "Dump counters on exit")
 DEFINE_bool(debugger, false, "Enable JavaScript debugger")
 DEFINE_bool(remote_debugger, false, "Connect JavaScript debugger to the "
                                     "debugger agent in another process")
 DEFINE_bool(debugger_agent, false, "Enable debugger agent")
 DEFINE_int(debugger_port, 5858, "Port to use for remote debugging")
-DEFINE_string(map_counters, NULL, "Map counters to a file")
+DEFINE_string(map_counters, "", "Map counters to a file")
 DEFINE_args(js_arguments, JSArguments(),
             "Pass all remaining arguments to the script. Alias for \"--\".")
 
 #if defined(WEBOS__)
 DEFINE_bool(debug_compile_events, false, "Enable debugger compile events")
 DEFINE_bool(debug_script_collected_events, false,
             "Enable debugger script collected events")
 #else
 DEFINE_bool(debug_compile_events, true, "Enable debugger compile events")
 DEFINE_bool(debug_script_collected_events, true,
@@ skipping 169 matching lines @@
 #undef FLAG
 
 #undef DEFINE_bool
 #undef DEFINE_int
 #undef DEFINE_string
 
 #undef FLAG_MODE_DECLARE
 #undef FLAG_MODE_DEFINE
 #undef FLAG_MODE_DEFINE_DEFAULTS
 #undef FLAG_MODE_META