Fix concurrent access to VMState::current_state_.

src/vm-state.cc

 // 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 15 matching lines @@
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include "v8.h"
 
 #include "vm-state.h"
 
 namespace v8 {
 namespace internal {
 
 #ifdef ENABLE_VMSTATE_TRACKING
-VMState* VMState::current_state_ = NULL;
+AtomicWord VMState::current_state_ = 0;
 #endif
 
 } }  // namespace v8::internal