Oilpan: IDL union objects should be put in HeapVector

Source/bindings/templates/union.h

Index: Source/bindings/templates/union.h
diff --git a/Source/bindings/templates/union.h b/Source/bindings/templates/union.h
index 497846bc30da6b7236e64e66679615a78fc64fad..4e66ce49c22ad08b8b0fdda35f38b7cbb5581691 100644
--- a/Source/bindings/templates/union.h
+++ b/Source/bindings/templates/union.h
@@ -89,4 +89,12 @@ public:
 {% 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}}