[bindings] Pass v8::Local<v8::Value> by value in NativeValueTraits<T>::nativeValue()

Source/bindings/templates/union.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file has been auto-generated by {{code_generator}}. DO NOT MODIFY!
 
 #include "config.h"
 #include "{{header_filename}}"
 
 {% from 'conversions.cpp' import v8_value_to_local_cpp_value %}
@@ skipping 166 matching lines @@
     {% for member in container.members %}
     case {{container.cpp_class}}::{{member.specific_type_enum}}:
         return {{member.cpp_value_to_v8_value}};
     {% endfor %}
     default:
         ASSERT_NOT_REACHED();
     }
     return v8::Local<v8::Value>();
 }
 
-{{container.cpp_class}} NativeValueTraits<{{container.cpp_class}}>::nativeValue(const v8::Local<v8::Value>& value, v8::Isolate* isolate, ExceptionState& exceptionState)
+{{container.cpp_class}} NativeValueTraits<{{container.cpp_class}}>::nativeValue(v8::Local<v8::Value> value, v8::Isolate* isolate, ExceptionState& exceptionState)
 {
     {{container.cpp_class}} impl;
     V8{{container.cpp_class}}::toImpl(isolate, value, impl, exceptionState);
     return impl;
 }
 
 {% endfor %}
 } // namespace blink