Adds a --help option and usage message listing all flags to V8.

src/flag-definitions.h

 // Copyright 2008 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 182 matching lines @@
               "file in which to testing_serialize heap")
 #else
 DEFINE_string(testing_serialization_file, "/tmp/serdes",
               "file in which to serialize heap")
 #endif
 
 //
 // Dev shell flags
 //
 
+DEFINE_bool(help, false, "Print usage message, including flags, on console")
 DEFINE_bool(dump_counters, false, "Dump counters on exit")
 
 //
 // Debug only flags
 //
 #undef FLAG
 #ifdef DEBUG
 #define FLAG FLAG_FULL
 #else
 #define FLAG FLAG_READONLY
@@ skipping 104 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