[bindings] Make ScriptValue::v8ValueFor(...) a 'const' method.

Source/bindings/core/v8/ScriptValue.h

 /*
  * Copyright (C) 2008, 2009 Google Inc. 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
@@ skipping 154 matching lines @@
     {
         m_value = nullptr;
     }
 
     v8::Local<v8::Value> v8Value() const;
     // TODO(bashi): Remove v8ValueUnsafe().
     v8::Local<v8::Value> v8ValueUnsafe() const;
     // Returns v8Value() if a given ScriptState is the same as the
     // ScriptState which is associated with this ScriptValue. Otherwise
     // this "clones" the v8 value and returns it.
-    v8::Local<v8::Value> v8ValueFor(ScriptState*);
+    v8::Local<v8::Value> v8ValueFor(ScriptState*) const;
 
     bool toString(String&) const;
 
     void setReference(const v8::Persistent<v8::Object>& parent, v8::Isolate* isolate)
     {
         m_value->setReference(parent, isolate);
     }
 
     static ScriptValue createNull(ScriptState*);
 
 private:
     RefPtr<ScriptState> m_scriptState;
     RefPtr<SharedPersistent<v8::Value>> m_value;
 };
 
 } // namespace blink
 
 #endif // ScriptValue_h