Clean up VMState a little bit.

src/vm-state.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 16 matching lines @@
 
 #ifndef V8_VM_STATE_H_
 #define V8_VM_STATE_H_
 
 #include "allocation.h"
 #include "isolate.h"
 
 namespace v8 {
 namespace internal {
 
+template <StateTag Tag>
 class VMState BASE_EMBEDDED {
  public:
-  inline VMState(Isolate* isolate, StateTag tag);
+  explicit inline VMState(Isolate* isolate);
   inline ~VMState();
 
  private:
   Isolate* isolate_;
   StateTag previous_tag_;
 };
 
 
 class ExternalCallbackScope BASE_EMBEDDED {
  public:
   inline ExternalCallbackScope(Isolate* isolate, Address callback);
   inline ~ExternalCallbackScope();
  private:
   Isolate* isolate_;
   Address previous_callback_;
 };
 
 } }  // namespace v8::internal
 
 
 #endif  // V8_VM_STATE_H_