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

src/mksnapshot.cc

Index: src/mksnapshot.cc
===================================================================
--- src/mksnapshot.cc	(revision 701)
+++ src/mksnapshot.cc	(working copy)
@@ -150,10 +150,10 @@
   // Print the usage if an error occurs when parsing the command line
   // flags or if the help flag is set.
   int result = i::FlagList::SetFlagsFromCommandLine(&argc, argv, true);
-  if (result > 0 || argc != 2 || i::FLAG_h) {
+  if (result > 0 || argc != 2 || i::FLAG_help) {
     ::printf("Usage: %s [flag] ... outfile\n", argv[0]);
     i::FlagList::PrintHelp();
-    return !i::FLAG_h;
+    return !i::FLAG_help;
   }
 
   v8::V8::SetCounterFunction(counter_callback);