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

Source/bindings/core/v8/ScriptValue.cpp

 /*
  * Copyright (C) 2008, 2009, 2011 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 44 matching lines @@
     return m_value->newLocal(isolate());
 }
 
 v8::Local<v8::Value> ScriptValue::v8ValueUnsafe() const
 {
     if (isEmpty())
         return v8::Local<v8::Value>();
     return m_value->newLocal(isolate());
 }
 
-v8::Local<v8::Value> ScriptValue::v8ValueFor(ScriptState* targetScriptState)
+v8::Local<v8::Value> ScriptValue::v8ValueFor(ScriptState* targetScriptState) const
 {
     if (isEmpty())
         return v8::Local<v8::Value>();
     v8::Isolate* isolate = targetScriptState->isolate();
     if (&m_scriptState->world() == &targetScriptState->world())
         return m_value->newLocal(isolate);
 
     ASSERT(isolate->InContext());
     v8::Local<v8::Value> value = m_value->newLocal(isolate);
     RefPtr<SerializedScriptValue> serialized = SerializedScriptValueFactory::instance().createAndSwallowExceptions(isolate, value);
@@ skipping 12 matching lines @@
     result = toCoreString(v8::Local<v8::String>::Cast(string));
     return true;
 }
 
 ScriptValue ScriptValue::createNull(ScriptState* scriptState)
 {
     return ScriptValue(scriptState, v8::Null(scriptState->isolate()));
 }
 
 } // namespace blink