Oilpan: IDL union objects should be put in HeapVector

Source/bindings/templates/union.h

 {% include 'copyright_block.txt' %}
 #ifndef {{macro_guard}}
 #define {{macro_guard}}
 
 {% for filename in header_includes %}
 #include "{{filename}}"
 {% endfor %}
 
 namespace blink {
 
@@ skipping 71 matching lines @@
         {# 1. null or undefined #}
         if (isUndefinedOrNull(v8Value))
             return;
         V8{{cpp_type}}::toImpl(isolate, v8Value, impl, exceptionState);
     }
 };
 
 {% endfor %}
 } // namespace blink
 
+// We need to set canInitializeWithMemset=true because HeapVector supports
+// items that can initialize with memset or have a vtable. It is safe to
+// set canInitializeWithMemset=true for a union type object in practice.
+// See https://codereview.chromium.org/1118993002/#msg5 for more details.
+{% for container in containers %}
+WTF_ALLOW_MOVE_AND_INIT_WITH_MEM_FUNCTIONS(blink::{{container.cpp_class}});
+{% endfor %}
+
 #endif // {{macro_guard}}