Make classes for union types exported.

Source/bindings/templates/union.cpp

 {% from 'conversions.cpp' import declare_enum_validation_variable %}
 // 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}}"
 
@@ skipping 40 matching lines @@
 }
 
 {{container.cpp_class}} {{container.cpp_class}}::from{{member.type_name}}({{member.rvalue_cpp_type}} value)
 {
     {{container.cpp_class}} container;
     container.set{{member.type_name}}(value);
     return container;
 }
 
 {% endfor %}
+#if COMPILER(MSVC) && defined(COMPONENT_BUILD) && LINK_CORE_MODULES_SEPARATELY
+{{container.cpp_class}}::{{container.cpp_class}}(const {{container.cpp_class}}&) = default;
+{{container.cpp_class}}::~{{container.cpp_class}}() = default;
+{{container.cpp_class}}& {{container.cpp_class}}::operator=(const {{container.cpp_class}}&) = default;
+#endif
+
 {% if container.needs_trace %}
 DEFINE_TRACE({{container.cpp_class}})
 {
     {% for member in container.members if member.is_traceable %}
     visitor->trace(m_{{member.cpp_name}});
     {% endfor %}
 }
 
 {% endif %}
 void V8{{container.cpp_class}}::toImpl(v8::Isolate* isolate, v8::Local<v8::Value> v8Value, {{container.cpp_class}}& impl, ExceptionState& exceptionState)
@@ skipping 115 matching lines @@
 
 {{container.cpp_class}} NativeValueTraits<{{container.cpp_class}}>::nativeValue(v8::Isolate* isolate, v8::Local<v8::Value> value, ExceptionState& exceptionState)
 {
     {{container.cpp_class}} impl;
     V8{{container.cpp_class}}::toImpl(isolate, value, impl, exceptionState);
     return impl;
 }
 
 {% endfor %}
 } // namespace blink