Don't allow setting properties on scriptable v8 plugin backing objects

Source/bindings/core/v8/custom/V8HTMLPlugInElementCustom.cpp

 /*
 * Copyright (C) 2009 Google Inc. All rights reserved.
 * Copyright (C) 2014 Opera Software ASA. 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.
@@ skipping 82 matching lines @@
         return;
 
     // We need to directly call setter on NPObject to be able to detect
     // situation where NPObject notifies it does not possess the property
     // to be able to lookup standard DOM property.
     // This information is lost when retrieving it through v8::Object.
     if (isWrappedNPObject(instance)) {
         callNpObjectSetter(instance, property, value, info);
         return;
     }
-
-    if (instance->Set(property, value))
-        v8SetReturnValue(info, value);
 }
 } // namespace
 
 void V8HTMLAppletElement::namedPropertyGetterCustom(v8::Local<v8::String> name, const v8::PropertyCallbackInfo<v8::Value>& info)
 {
     getScriptableObjectProperty<V8HTMLAppletElement>(name, info);
 }
 
 void V8HTMLEmbedElement::namedPropertyGetterCustom(v8::Local<v8::String> name, const v8::PropertyCallbackInfo<v8::Value>& info)
 {
@@ skipping 86 matching lines @@
     UseCounter::count(V8HTMLEmbedElement::toNative(info.Holder())->document(), UseCounter::HTMLEmbedElementLegacyCall);
 }
 
 void V8HTMLObjectElement::legacyCallCustom(const v8::FunctionCallbackInfo<v8::Value>& info)
 {
     invokeOnScriptableObject<V8HTMLObjectElement>(info);
     UseCounter::count(V8HTMLObjectElement::toNative(info.Holder())->document(), UseCounter::HTMLObjectElementLegacyCall);
 }
 
 } // namespace blink