Serializer enable/disable flags need thread safety.

src/flags.cc

 // Copyright 2006-2008 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 527 matching lines @@
   for (size_t i = 0; i < num_flags; ++i) {
     flags[i].Reset();
   }
 }
 
 
 // static
 void FlagList::PrintHelp() {
 #if V8_TARGET_ARCH_ARM
   CpuFeatures::PrintTarget();
-  CpuFeatures::Probe();
+  CpuFeatures::ProbeWithoutIsolate();
   CpuFeatures::PrintFeatures();
 #endif  // V8_TARGET_ARCH_ARM
 
   printf("Usage:\n");
   printf("  shell [options] -e string\n");
   printf("    execute string in V8\n");
   printf("  shell [options] file1 file2 ... filek\n");
   printf("    run JavaScript scripts in file1, file2, ..., filek\n");
   printf("  shell [options]\n");
   printf("  shell [options] --shell [file1 file2 ... filek]\n");
@@ skipping 13 matching lines @@
 
 
 // static
 void FlagList::EnforceFlagImplications() {
 #define FLAG_MODE_DEFINE_IMPLICATIONS
 #include "flag-definitions.h"
 #undef FLAG_MODE_DEFINE_IMPLICATIONS
 }
 
 } }  // namespace v8::internal